The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity accidentally introduced into its body; a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk. Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre. Nacre is composed chiefly of calcium carbonate. Because very few natural pearls are now on the market, most pearls used in fine jewelry are cultured. These include "Biwa" pearls and most other freshwater pearls. Cultured pearls are not easily distinguished from natural pearls except by an expert.

What is the chief component of nacre

A．Sand.

B．Bead.

C．Calcium carbonate.

D．Biwa.

家务劳动时间缩短的主要途径包括（）

A.减少家务劳动

B.增加闲暇时间

C.增加工作时间

D.家务劳动机械化

E.家务劳动社会化