业务工作底稿用对照数字的方式进行索引，索引的主要目标是：

A．允许交叉索引并简化监督性复核。
B．支持最终的业务沟通。
C．消除后续追踪复核的需要。
D．确定工作底稿恰当支持业务观察、结论和建议。

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.

Cultured pearl is formed by________.

A．insertion of a pearl into a live mollusk

B．an oyster into which a piece of grit has been placed

C．putting in a live mollusk

D．placing a bead into culture