You should have helped him ______ you were busy.

[　　]

He didn't ______ school yesterday because he had to ______ his sick mother.

[　　]

The professor has received the letter ______ to give a lecture in Princeton Institute.

[　　]

The whole book ______ 12 units, ______ two mainly revisions.

[　　]

--Your phone number again? I ________ quite catch it.

--It's 8453228.

[　　]

I first met Lisa three years ago. She _________ at a radio shop at the time.

[　　]

A．has worked

B．was working

C．has been working

D．had worked

Although the causes of cancer _________, we do not yet have any practical way to prevent it.

[　　]

A．are being uncovered

B．have been uncovering

C．are uncovering

D．have uncovered

Tom, you ______ leave all your clothes on the floor like this!

The World Wide Web is sometimes jokingly called the World Wide Wait because it ______ be very slow.

John, look at tile time. ______ you play the piano at such a late hour?