When you are _______ about which button to press on the complicated machine, you can either read the user’s guide for guidance or fax us for specific instruction. Don’t ______ the trial-and-error method.[ ]

Between the two generations, it is often not their age, _________ their education that causes misunderstanding

We all know trees play an important role in _______up the air.

You will fall behind others ______ you work hard.

Sorryfor what I have said to you． —_____． A．Forget it B．You are nice C．Don’t mention it D．You’d better not