问题

service ads, also PSAs, often _ for free, _ to educate people about health, safety or any other problem that affects public welfare

A:that
B:which
C:不填
D:what

正确答案是：

同类题型

Do you think yesterday’s math problem was difficult— Yes, but I worked it out.

We can’t go on with the experiment because the computer_________

Recently quite a lot of experts havesuggested that another law on wildlife protectionas soon as possible．

－ ________ does the bridge made of steel weigh －About 90,000 tons.

How are you feeling now — Well, I’m feeling _____________ to go out for a walk.