The letter is from my sister is working in Beijing. A. which B. that C. whom D. who

问题补充：

The letter is from my sister, is working in Beijing.

A. which

B. that

C. whom

D. who

答案：

A. which

B. that

C. whom

(答案→)D. who

解析：who引导非限制性定语从句,who作从句的主语.