According to this excerpt why was the act necessary?

20 points “the creation of a mosaic was a complicated procedure that called for careful timing and teamwork. . walls were painted with resin or tar before being covered with the first of three layers of plaster. . a sketch of the projected work was then made on the plaster to serve as a guide for the various artisans. the outermost layer of plaster, in which the pieces, or tesserae, of the mosaic would be set, was spread on only enough area to contain a day’s work. some of the tesserae—small, carefully cut fragments of colored glass, marble, and semiprecious stones—were pressed into the wet plaster so that they projected at a slight angle. the stones thereby better reflected light and imbued the finished mosaic with a shimmering life of its own.” —empires besieged: time frame a.d. 200–600 how many layers of materials were under the tiles? a. two- c. four- b. three- d. five