问题补充：
When the fire broke out, Mary didn know .
A. what to do
B. what do
C. to do what
D. what thing to do
答案：
(答案→)A. what to do
B. what do
C. to do what
D. what thing to do
解析：Mary didn know what to do意为: Mary不知道该做些什么? what to do作know的宾语.
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