“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

Which sentence of this passage best states the main idea?
a.
first
c.
fifth
b.
third
d.
seventh