He paid for a seat,when he ___ have enter free.A .could B .would C .must D .need

问题描述:

He paid for a seat,when he ___ have enter free.
A .could B .would C .must D .need

选择A.
could have done 虚拟语气,译:本可以怎样怎样,但是没怎样怎样.