单选题 Mr. Grey has two sons ______ are famous writers.
[A] they both [B] both of whom
[C] both of them [D] both they

【正确答案】 B

【答案解析】[解析] 本题考查非限制性定语从句。原句中，两个单句之间没有连词，所以应该是主句和从句的关系。both of whom引导的非限制性定语从句，来补充说明先行词sons。因此本句的答案是[B]，意思是“他有两个儿子，(这两个儿子)都是有名的医生”。