—David is often absent from class.—Tell him he ________ answer for it if he goes on behaving like that.

—Hurry up , or you ________ catch the train. 

Trees ____ fight air pollution．They are natural air conditioners．

The boy is old enough to ---------himself.

. (2010·江西名校模拟) —I’ve heard they went skiing in the mountains last winter.                           —It ______ true because there was so little snow there then.