The city walls _______ forty years ago. What a pity!
—Have all the students known that our class will visit the factory this afternoon?—Yes. Every student______ about it.
--Sorry, I overslept. I forgot to set the alarm.--Your clock ________. Perhaps you should buy a new one.
--There is somebody knocking at the door. Who________ it be?--I’m not sure. It ________ be a postman.
----How do you find your new classmates ?--Most of them are kind, but _________is so good to me as Bruce.
--Do you think the math problem was difficult?--Yes. But I could ________ make it.