问题补充：
When I went to their house, there was no light on. They at home.
A. can’t be
B. couldn’t be
C. can’t have been
D. mustn’t have been
答案：
A. can’t be
B. couldn’t be
(答案→)C. can’t have been
D. mustn’t have been
解析：句意：当我到达他们家的时候，没有灯光。他们不可能在家。can’t have been不可能发生了某事，情态动词表示推测，用于对过去发生的事情的否定推测，故选C
