—Mr. Black, may I leave now?—Well, if your work ________, you can go home.

You have to stay behind if your homework         before five o'clock.

—Oh, this is really an old building.—Do you know when it        ?

— At present, one of the best ways to study is working in groups.—More chances _______ to students to learn from each other.

—It’s difficult to get to the other side of the river.—I think a bridge        over the river.