有读者问这样一道选择题：
(1)My pain______________apparent the moment I walked into the room, for the first man I met asked sympathetically, “Are you feeling all right?”
A. must be B. had been
C. must have been D. had to be
答案是C，我认为A也对，你意如何？
“must have+过去分词”是一完成时态，它可表示的动作或状态必须发生或存在于过去。上述句（1）之所以应选must have been就是这个道理。再如：
(2)He must have attended the meeting yesterday.
(3)She must have been very young when she got married.
must be亦可用以表过去存在的状态，但多用于间接引语中，如：
(4)He thought she must be tired.
(5)She was sure that he must be Peter.
有时，must be形式上并不用在间接引语中，但实质上（即从上下文看）仍是用在间接引语中，如：
(6)There was nobody else in the changing-room. They must all be in the hotel already.
因此，上述句（1）不可选用must be。
