A: is Mary?B: I think she is at homeA. WhoB. WhereC. WhatD. How

问题补充：

A: is Mary?

B: I think she is at home．

A. Who

B. Where

C. What

D. How

答案：

A. Who

(答案→)B. Where

C. What

D. How

解析：疑问词辨析。Who谁；Where在哪儿；What什么；How怎样。

根据答语的内容I think she is at home，我认为他在家呢。可知上文提问地点，故选B