Question

A: _____is Mary?
B: I think she is at home．

1. A.
   Who
2. B.
   Where
3. C.
   What
4. D.
   How

Answer 1

B
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