I couldnt have got to the meeting on time - an earlier train . A. if I had not caughtB.

I couldn't have got to the meeting on time - an earlier train .
A. if I had not caught
B. unless I had caught
C. if I did not catch
D. unless I caught

A. if I had not caught(答案→)B. unless I had caught C. if I did not catchD. unless I caught解析：if…not和unless有时可以互相替换，但也有不能互相替换的时候。unless用于这样的句子，即如果A不受阻于B，A将发生，例如：He will accept the job if the salary is not too low / unless the salary is too low . (如果薪水不太低，他就会接受这一工作/除非薪水太低，否则他会接受这一项工作的。)但是unless不能用于由于未发生B而发生A的句子里，如：I will be quite glad if she does not come this evening (她今天晚上要是不来，我才高兴呢。)又如：I will be surprised if he does not win the game next week . (如果他赢不了下星期那场比赛，我倒会感到惊奇的。)在以上的两个句子中不能使用unless，但是unless常用来引导一个谈及过去的事后的想法，unless从句跟着主句，通常用破折号而不是逗号将它与主句分开。第7题的句意是：我不可能准时到会了-除非我当时赶上更早一列火车。这个句子的实际是说：我没有准时到会。我只有赶上更早的一趟火车，才能准时到会。如果用if … not代替上面句中的unless，那么这个句子就变成：I couldn't have got to the meeting on time if I hadn't caught an earlier train . (要不是我赶上了更早的一班火车，我就不可能准时到会。)这句话表达的意思与上句完全相反：我确实准时到会了，因为我赶上了更早的一趟火车。

这个解释是对的