Yesterday I didn’t _____ any seals but _____ some dolphins.

Mr. Wang __________ be in the office. The light is still on.—No, he __________ be. Because he phoned me from the farm just now.

The next thing _____must be done is to make a plan. [ ]

Once we discover a mistake, we should not ______ it but correct it immediately, to create better conditions for economic development.

______ his bike is lost? — He has to go to school on foot.