He has      for three or four days. 

－Is that Mary?－No, it _______be her. She has gone to Tokyo. 

Generally my father is a mild man, but when he gets angry, he ______ be fierce.

 —Why are you late again?—Something happened to our friend Wang Fei. I _____ to help him.

Cars play an important role in modern life, but they ______ also cause many problems like air pollution.