Question

Mr. Mike doesn’t understand ________ made his wife so upset this morning

1. A.
   what was it
2. B.
   why it was that
3. C.
   how that was
4. D.
   what it was that

Answer 1

D
understand 后面是一个宾语从句，从句中缺主语，同时又是强调结构的疑问形式用在宾语从句中。