Cafeteria tray with protections and method for its use Description Technical Field The present invention relates to the field of the manufacture of cafeteria trays made of plastic material, and in particular to cafeteria trays made of recycled plastics. More specifically, the object of the present patent application concerns cafeteria trays provided with two protections made of transparent recycled plastics, such as to isolate the user from the other diners. Background Art The Covid 19 pandemic has caused enormous damage to collective and commercial catering given the inability for various customary users to use this service, due to the legislation adopted for limiting the infections. Actually, the dining areas (canteens) are provided with tables for 4 or 6 persons, but in order to comply with safe distancing rules, these have been limited to only 1 or 2 seats. This implied a considerable reduction of diners’ affluence in dining areas, due to the lesser capacity of the latter. To date, there are, in fact, multiple solutions allowing customers to buy takeaway food and eat it somewhere else. As an example, take the Patent Application US5857583A of ANNETTE M. CHANTAKA and KEITH C. HIGGINSON. This invention concerns a container for catering services and comprises a tray with a cover, preferably made of moldable material which is absorbing and resistant to leaks such as molded fiber. The tray has a first body portion and a first edge which comprises and laterally protrudes – to the outside - from the first body portion. The first body portion has a main compartment for food preservation and at least a pocket for lodging/receiving the cups for the beverages. The pocket that contains the glasses forms flexible lateral wall portions allowing the pocket with the glasses to firmly retain/hold glasses of various sizes. The cover is hingedly connected to the tray. The cover includes a second body portion and a second edge that encloses and laterally projects to the outside from the second body portion. The cover is movable between an open position and a closed position. When the cover is in closed position, said cover covers the main compartment for food preservation. Another solution is suggested by the Patent Application US6932214B1 (DAVE ZIMET). The subject of this application relates to a disposable fast food box container with a pair of semicircular handles for convenient carrying and a drink holder therein. The handles include strap portions that are attached to the lid that allow a vehicle seat belt to be passed thereunder and serve to hold the lid in an open position close to the chest of the user. The lid includes a pair of side flaps that cooperate with the lid portion for directing any crumbs generated during eating into the box/container directly below thus eliminating the usual mess that occurs during eating in a vehicle. The drink holder includes a conical flexible wall portion that becomes inverted when engaged by the base of a drink container bringing the conical flexible wall into engagement with conical outer surface of the drink container to stabilize drink container therein. Despite the fact that they allow to eat the meal in total safety, the proposed solutions further limit the use of seats inside dining areas. An object of the present patent application is to suggest a means that allows, in a safe manner, to return to using the canteens/dining areas, by realizing a tray which is washable and reusable and which on the one hand is suited to act as a protection for the user and on the other forms a rapid and intuitive means for companies to efficiently reorganize their canteen services. Description of Invention According to the present invention, advantageously there is provided a both ecological and economical solution to the manufacture of cafeteria trays, by producing them in recycled plastics. These cafeteria trays made of recycled plastics, which are obtained from the same thermoplastic polymers as the usual cafeteria trays, are advantageously produced through a known manufacturing method and through a typical already existing production plant. Advantageously, these trays are also resistant to chemical substances and have a scratchproof and stain resistant surface. This cafeteria tray preferably presents a quadrangular shape and comprises: • an internal surface adapted to act as an abutment surface for foods and drinks, rendering the transport thereof easy and safe; • an edge in relief adapted to prevent the outward sliding of the content on the internal surface; said edge being installed along the perimeter of said internal surface; • a hollow lower shelf adapted to contain at least two protections made of transparent recycled plastics, when these are unused; said hollow lower shelf being situated immediately below said internal surface and comprising a base and tabs; • at least two of said tabs, adapted to stably block said protections made of transparent recycled plastics within the hollow lower shelf, when said protections made of transparent recycled plastics are unused; said tabs are installed at two opposite sides of the base of said hollow lower shelf through hinges; said tabs are shaped in a manner such to fill the space between the perimeters of said base of said hollow lower shelf and the perimeter of said internal surface of said tray, achieving the isolation of said lower shelf; • said hinges adapted to allow the opening and/or closing of said tabs; said hinges being installed preferably centrally of two sides of said base of said hollow lower shelf, where the tabs are installed; • at least three supports with internal slit adapted to firmly retain the protections made of transparent recycled plastics when these are used; said supports with slit being installed in number of at least one on each side of said edge in relief, excluding one of these sides; • at least three hollow housings shaped according to the reverse image of said supports with slit and installed below of said hollow lower shelf; said hollow housings being installed precisely aligned according to the position of said supports with internal slit, allowing a simple stacking of trays. This cafeteria tray made of recycled plastics is further provided with two protections in transparent recycled plastics, adapted to isolate the user from the other diners. Said protections formed of transparent recycled plastics are shaped like said tray and have a slightly minor dimension, so as to be engageable/introducible in said hollow lower shelf, when unused. The user, in order to reach an adequate safety, will have to use the device as follows: • engaging a protection in the front part, i.e. across from the user, by means of one of the supports with internal slit; • engaging another protection in a lateral part by means of another one of the supports with internal slit at the position of a nearby diner. Thus conceived, the effect would be to create a protective barrier around each user of the cafeteria service, so as to protect and isolate him/her from the other diners, and so that he or she can eat his/her meal in total safety, without employing any personal protective equipment whatever. In fact, each individual will be protected and/or isolated on all three sides; to the front, by means of one of the protections offered by the tray; on one of the two sides, with the second protection offered by the tray; on the side opposite the latter, by the protection used in turn by the person who sits alongside the user. Advantageously, the tray will be modular based on the needs of the diner who can select whether to use only one Plexiglas protection, both protections, or none (if at that moment there are no other people at the table). The present invention is of considerable utility even in other places, other than cafeterias, it being usable also by those who, due to the most diverse needs, are forced to eat at their own workplaces. The advantages offered by the present invention are obvious in the light of the description as outlined so far, and they will be even more clear thanks to the annexed figures and their detailed description. Description of Drawings The invention will now be described in at least one preferred embodiment for illustrative and non-limitative purposes with the help of the annexed Figures, in which: FIGURE 1 is a three-dimensional view which illustrates the invention from above when the protections are disengaged; FIGURE 2 is a three-dimensional view which illustrates the invention from above, when the protections are engaged; FIGURE 3 is a vertical sectional front view of the invention, when the protections are disengaged. Detailed Description of Invention The present invention will now be described only for illustrative but non-limitative or binding purposes with reference to the Figures, which represent one of the embodiments related to the present inventive concept. With reference to FIGS.1, 2 and 3 these show the main components of an example of cafeteria tray with protections 100. In FIG. 1 there is shown the device according to the present patent application, when the protections made of recycled plastics 80 are not used, whereas FIG. 2 shows the invention when said protections of recycled plastics 80 are engaged. FIG. 3 further allows to understand the function of the lower shelf 30 and of the opening/closing movement of the tabs 40. Said cafeteria tray with protections 100 includes: an inner surface 10 adapted to act as an abutment surface for foods and drinks, rendering the transport thereof easy and safe; an edge in relief 20 adapted to prevent the outward sliding of the content on the internal surface; said edge in relief 20 being installed along the perimeter of said internal surface 10; a hollow lower shelf 30 adapted to contain at least two transparent protections 80, when these are unused; said hollow lower shelf 30 being situated immediately below said internal surface 10 and comprising a base 90 and tabs 40; the said two tabs 40 adapted to stably block said protections 80 within the hollow lower shelf, when said protections 80 are unused; said tabs 40 being installed at the two short sides of said base 90 of said hollow lower shelf 30 through hinges 50; said tabs 40 being shaped in a manner such to fill the space between the perimeters of said base 90 of said hollow lower shelf 30 and the perimeter of said internal surface 10, achieving the isolation of said lower shelf 30; said hinges 50 adapted to allow the opening and/or closing of said tabs 40; said hinges 50 being installed centrally of the two short sides of said base 90 of said hollow lower shelf 30; three supports with internal slit 60 adapted to firmly retain the protections 80 made of transparent recycled plastics when these are used; said supports with slit 60 being installed on the three sides of said edge in relief 20, excluding one of the long sides; three hollow housings 70 shaped according to the reverse image of said supports with slit 60 and installed below of said hollow lower shelf 30; said hollow housings 70 being installed precisely aligned according to the position of said supports with internal slit 60, allowing a simple stacking of multiple cafeteria trays made of recycled plastics with protections 100; two protections 80 adapted to isolate the user from the other diners; said protections 80 being engaged/inserted in said hollow lower shelf 30, when unused, or being engaged/inserted on said supports with internal slit 60, when one decides to use them. In the present preferred embodiment said cafeteria tray 100 has a rectangular form of 50x35x5 cm. Said protections 80 are of the same shape as the cafeteria tray 100 and have slightly minor dimensions, so that they can be easily introduced in said hollow lower shelf 30, when these are unused. Said protections, when used, are engaged on said supports with internal slit 60 whose slits are of the same size as the thickness of said protections 80, thereby guaranteeing a firm retention. Said protections 80 are retained in a horizontal condition; therefore, taking advantage of the height (5cm) of said cafeteria tray 100, an adequate protection and/or isolation is ensured by reaching about 40 cm in height for each side on which said protections 80 are engaged. All components of said cafeteria tray with protections 100 are made of recycled plastics. Finally, it should be clear that the invention described above can be modified and that variants and additions, obvious to a skilled person, may be made, without departing in this way from the scope as defined in the appended claims.