-----We did not see him at the lecture yesterday.------You______him,since I went to an international conference with him,B.can't have seen C.couldn't see can't have done/shouldn't have done/mustn't have

A 是虚拟语气
情态动词＋ have done都是表示对过去动作的推测
must have done 过去一定做了某事
should /need have done 本来应该／需要做某事（却没做）
should't/needn't have done本来不应该／不需要做某事（却做了）
may/might have done 过去可能做过某事
could have done过去有能力做某事

B.can't have seen

can't have done/shouldn't have done都是对过去的虚拟,不能用mustn't have done表示对过去的猜测can't have done是过去不可能做shouldn't have done是本不应该做,但是已经做了选B是因为这是对过去的猜测.第二个人说...

正确答案是C。
can't/couldn't 不可能
本题真正的干扰项应该是couldn't have seen（基于过去时间的“不可能已经”，用来否定对方使用一般过去时态的描述或猜测）。
其它几个选项的语意只需在原有情态动词语意前面加上一个“本”字即可。