（题文）Optimism has a positive effect ________ people's physical and mental health, ________ which I'm very convinced.
A. on; of B. to; in
C. at; at D. on; at
（题文）The problem ______ tomorrow is important.
A. will be discussed B. discussed
C. being discussed D. to be discussed
We must find out ________ Karl is coming, so we can book a room for him.
A. how B. why
C. when D. where
He had nothing to do but ______ at the bus-stop in the rain.
A. to wait B. wait
C. waiting D. should wait
It is good manners to eat up ______ is left in your plate.
A. that B. what
C. which D. all
With time ______ by, my grandmother has much more silver hair.
A. went B. goes
C. going D. gone
