___ he did it remains a secret.            

A. What       B. Whom       C. Which       D. How

The scientist is well known us his great achievement．

A．to；for B．by；for C．by；as D．to；in

We didn’t live together. We kept in touch _______ writing often.

A. with    B. of  C. on     D. by

There are altogether eleven books on the shelf, ______five are mine.

      A.on which          B. in which           C.of which          D. from which

You will find friendly people ___ you go in China.

A. where              B. and                C. wherever          D. so

Please write me ____ you arrive in New York.

A. as well as                  B. so long as            C. as far as            D. as soon as

He decided to the matter himself．

A．look through B.look into C．look after D．look up

_______ one old lady, the bus was empty.

A. Except B. Except for  C. But      D. Besides

My sister is expecting me,____ I must be off now.       

A. however           B. or              C. so             D. otherwise

He is very old,____ he still works very hard.     

A. but               B.if            C.when             D.as