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.
单选题
Which of the following people could tell the difference between a cultured pearl and an organic pearl?
【正确答案】
C
【答案解析】[分析] 细节题型。
见文章最后一句：Cultured pearls are not easily distinguished from natural pearls except by an expert．只有专家才能区分出天然珍珠和人工培育的珍珠。珠宝商当然应是这类专家，所以选项C正确。A．Scuba diver戴水肺的潜水员B．Fisherman渔民和D．Clerk职员当然不是鉴别珠宝的行家了。
单选题
What is the chief component of nacre?
【正确答案】
C
【答案解析】[分析] 细节题型。
见文章第四句：Nacre is composed chiefly of calcium carbonate．选项C．Calcium carbonate(碳酸钙)正确。
单选题
The difference between a pearl and a cultured pearl is the nature of the ______.
【正确答案】
B
【答案解析】[分析] 细节题型。
文章第二句(A pearl is the response of a marine…a live mollusk. )指出：天然珍珠中的刺激性的杂物是偶然进入体内的(accidentally introduced into its body)；而人工培育的珍珠中的杂物是人为加入的(the intentional insertion)；所以选项B正确。
单选题
Nacre is a substance that is ______.
【正确答案】
C
【答案解析】[分析] 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。
【答案解析】[分析] 细节题型。
文章第二句(…a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk．)指出：人工培育的珍珠是在活的贝类中人为加入珍珠母珠(the intentional insertion of a mother-of-pearl bead)；所以选项B．an oyster into which a piece of grit(粗沙)has been placed正确。