答案：本句只能用 must have been，不能用 must be。

分析：因为本句是指对过去情况的推测，所以必须要用“情态动词+动词的完成式”。

补充：又如以下各句也要用 must have been 而不用 must be：

Sorry I made such a fool of myself last night. I must have been drunk. 对不起，我昨晚真是出丑了，我一定是喝醉了。

Although the shock must have been great, her face remained inexpressive. 尽管受到的震惊一定很大，她仍面无表情。