There are a lot of computers in the lab,and they need professional __________ to make sure they can be in a good state.
| A．operation | B．maintenance | C．service | D．advice |
—I have been waiting for you so long.
—Sorry, I have been in the library, _______ my friend look for some books.
| A．helped | B．helping | C．to help | D．help |
I think humour is a vital form of entertainment, ____ it makes us laugh and laughing keeps us healthy.
| A．when | B．after | C．if | D．because |
The UN is an important international organization _____helps many people around the world.
| A．who | B．that | C．where | D．/ |
When you get on stage for the first time, you may be surprised by ______bright the lights are.
| A．what | B．so | C．how | D．too |
If you had joined the chat room ten minutes ago, you ______what we were talking about!
| A．have known | B．knew | C．would have known | D．would know |