单选题 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

【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。