There is someone knocking at the door.It may ________ (be) the postman.如题

问题描述:

There is someone knocking at the door.It may ________ (be) the postman.
如题

有人在敲门,可能是邮递员。

be

be (情态动词后跟动词原形)有人在敲门,可能是邮递员