Peter ______ be really difficult to deal with at times even though he’s a nice person in general.

—May I smoke here ?—If you ____, choose a seat in the smoking section. 

If you have something important to do, you ___ waste any time.

（原创题）The patient insisted that he _____ ill, and _____back home at once.

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