what do you think make Mary so sad?___in the exam ,i think

题目

what do you think make Mary so sad?___in the exam ,i think
A.as she failed B.because of her failing C.failed D.failing

答案

D
(what do you think) make Mary so sad.
问句中提问部分是做主语,所以回答句选D,动名词做主语.