He paid for a seat, when he ______ have entered free

He paid for a seat, when he ______ have entered free

A.could

B.would

C.must

D.need

答案及解析： A

解析：

考查情态动词用法。本题先考虑本句可能出现的语境，设想一下某人去电影院看电影，本可以免费进入，却买了票。 could表示“可以”。