参考答案：B

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。