He paid for a seat,when he ________ have entered free．— something ________ to him．A.must happenB.should have happenedC.could have happenedD.must have happened
网友回答
A解析考查情态动词用法。could have done本能做而未做，would have done虚拟语气，must have done对过去事情的推测，“一定……”，句意为：他在本可以免费的情况下花了钱。