试题题干
Peter pulled up his collar so high that you（） have seen his head from behind if you had been there.
参考答案
正确答案：
试题解析
本句意为Peter把领子拉的很高，即便你在那，从后面也不可能看见他的头。couldn't have done不可能，shouldn't have done 本不应该做却做了，mustn't have done不存在这种形式，couldn't have done才是must have done 的否定形式。wouldn't have done是虚拟语气，指不会这么做。