--You ___________to be here an hour ago, but you’ve just arrived.-- Oh, I’m sorry for that.
The rubbish _____ is produced every day is becoming a serious problem in cities around the world.
More and more trees were cut down. _____, many animals are dying out.
We should often keep _____ touch with each other, because we are good friends.
—Do you think a radio is useful in learning English? —Sure. A radio can _____ listening English songs and reports.
—Do you know _____? —Sorry, I don’t know.