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  3. The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity

The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity

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问题     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 free 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.  
Nacre is a substance that is ______.
选项 A、mechanically manufactured
B、the result of laboratory testing
C、organically secreted by the mollusk
D、present in the chemical composition of freshwater pounds
答案C
解析 推断题型。文中第三句(Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre．)说明无论是天然珍珠还是人工：养殖的珍珠，它们的形成过程是一样的：贝类用一种被称之为珍珠质的物质将刺激性的杂物包裹住。由此推断出珍珠质nacre是贝类the mollusk滋生分泌出的，所以选项C正确。
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