Pollution is a serious problem. Scientists have _____ some good ways to save it.

.—I’d like to book a room for tonight.—Sorry, sir, but we don’t have any rooms______at the moment.

This is the same watch ______ I left on the playground yesterday.— How can it be possible? I bought _____ the other day

Does Jack _____ stay at home on weekends—-Yes, he _____ finish his homework first.

Having retired(退休), he ______ the club for old men and plays chess every day there.