B
因为裤子有两个裤管.
注意下述例句的单复数：
The trousers are blue.
The pair of trousers is blue.(前有a pair of,a kind of这类词修饰,谓语动词用单数）The pair of trousers is blue，前面是the不是a怎么还用is呢a pair of,the pair of都行，前者泛指，后者特指而已除了trousers常见的还有glasses