The air in big cities _____very dirty byfactories

When did you become a volunteer for the elderly——Two years ago. I ______ this group since then.

I didn’t go to the movie last nightbecause of the rain.– What a pity! If I had got the ticket, Iit.

The next morning there was always something in anyone else’s stockings. Only mine hung there __________.

Scientists are convinced _____ the positive effect oflaughter _____ physical and mental health．