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

【正确答案】 D

【答案解析】[解析] 本题考查非限定性定语从句。原句中，两句之间没有连词，应是主从关系。故要接both of whom引导的非限制性定语从句，whom作介词of的宾语，sons是关系代词的先行词。