—Is the letter from your mother?
—Yes. But she the letter herself, because she doesn't even know one word.
A. needn't have written
B. can't have written
C. might not have written
D. should not have written
网友回答
A. needn't have written
(答案→)B. can't have written
C. might not have written
D. should not have written
解析：选B 考查情态动词。句意：“这封信是你母亲来的吧？”“是的。但这封信不可能是她自己写的，因为她一个字也不认识。”根据because引导的原因状语从句可知，此处应用can't have done表示对过去发生的事很有把握的否定推测。