问题补充：
— I phoned you yesterday morning. A girl answered, but I didn’t recognize the voice.
— Oh, it my younger sister. She was in my room at that moment.
A. must have been
B. should have been
C. could have been
D. may have been
答案：
(答案→)A. must have been
B. should have been
C. could have been
D. may have been
解析：句意：—昨天上午我给你打电话。一个女孩的回答，但我没听出谁的声音。—哦，那一定是我妹妹。她那一刻正在我的房间里。情态动词加动词的现在完成式形式，表示对过去的推测。A. must have been表示一定做了某事。用于肯定推测；B. should have been本应该，表示应该做而没有做某事。C. could have been用于肯定句时，表示可能做了某事。更常用于否定推测；D. may have been 可能做过某事。表示可能性很小。结合语境判断，A最佳。
