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

The maths teacher told the naughty boys _____ so much noise in class.

The joke was so interesting that we all burst out laughing. Never before __________ heard a joke like that.

Is that your watch— ________. It’s his watch.

Take your time-it’s just_____short distance from hereto_____restaurant

The boy was put into the prison for ofstealing a lot of knives.