Question

I didn't hear the phone. I _____ asleep.

A. must be
B. must have been
C. should be
D. should have been

Answer 1

选B. must have been
解析：
本题考察对过去的猜测。
句子意思是：我没听到电话响。我肯定是睡着觉了。
根据I didn't hear the phone可以判断这个是发生在过去的事，
表示对过去的虚拟/猜测用could/might/must/should/would have done.
由于前面已经说明I didn't hear the phone（我没听到电话响），所以这里的语气应该是比较肯定，所以选must have been.
should have done意思是 本应该做的事没做。

Answer 2

答案B
B项是对过去事的肯定的猜测。