Scientists are convinced ___________the positive effect of laughter_________ physical andmental health.[ ]

I could hardly recognize your son. ________ I met him，he was only a little child.[ ]

I have two brothers. One is a worker, ________ is a driver.

She understood what I was talking about, ____ it was the first time we had spoken together

Susan, why are you still here They are all ready to start.—I’m sorry, but I _________ when to start.