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.

The difference between a pearl and a cultured pearl is the nature of the________.

A．color

B．introduction of the irritating impurity

C．coating material

D．irritating impurity

某企业投保企业财产保险综合险，固定资产保险金额为1000万元，在保险期内由于遭到泥石流而发生部分损失，造成房屋建筑损失为50万元，机器设备损失50万元，汽车损失50万元，不计残值。如果出险时固定资产的保险价值为2000万元，那么保险人对该损失的赔款是（）。

A.50万元

B.75万元

C.100万元

D.150万元