问题补充：
The old lady didn’t know when the house caught fire.
A. how to do
B. how to do with it
C. what to do
D. what to do it
答案：
A. how to do
B. how to do with it
(答案→)C. what to do
D. what to do it
解析：考查疑问词+不定式作宾语。此题中what+to do形成固定短语做know的宾语。句意：当她的房子着火的时候这位老妇人不知道做什么。所以选C。
