Question

2. The couple        debts in such a short time, for they are heavily in debt. (pay)

这对夫妻不可能在那么短的时间内还清了债务，因为他们现在都还在欠债呢。 

Answer 1

2. can't / couldn't have paid off考查情态动词的用法。从本句汉语提示看出，表达在过去没能还清债务，是对过去动作的推测。否定结构用can't / couldn' t have done。其他类似结构有： should have done本应该做(却没做).含有责备的语气，could have done本能够/可以做(却没有做).needn' t have done本没必要做(却做了 )' shouldn't have done本不应该做(却做了 ), might have done本可能做(却没有做)或有可能做过某事，must have done一定做过某事。