－Look, your sister is playing the piano over there.－She ______ be my sister. She has gone to America.

The problem is too difficult for him. He _______ solve it by himself without any help.

There is no need ______ because Mr. Smith is still sleeping.

According to the traffic rules, you _________ wear the safe belt while you are driving.

Nanjing isn’t so large __________ Shanghai, however, it’s the second __________ city in East China.