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—How long will it be ________ the professor makes another visit to China?

—It all depends.

A．until 

B．before

C．since 

D．when

“Sorry, sorry...”，he whispered, ________ talking to himself.

A．while 

B．even if

C．as if 

D．when

Scientists say it may be one year or so ________ it is possible to test the new medicine on human patients.

A．when 

B．after

C．before 

D．since

He had to be called two or three times ________ he would come to his dinner.

A．while 

B．as

C．before 

D．until

There have been fewer accidents on this highway ________ the speed limit was introduced.

A．before 

B．while

C．after 

D．since

—Our holiday  cost a lot of money!

—Did it? Well, that doesn't matter________you enjoyed yourselves.

A．as long as 

B．unless

C．as soon as

D．though

—How about camping this weekend，just for a change?

—OK，________you want.

A. whichever 

B. however

C. whatever 

D. whoever

________they decide which college to go to, students should research the admission procedures.

A. As        

B. While  

C. Until         

D. Once

The girl had hardly rung the bell______the door was opened suddenly，and her friend rushed out to greet her.

A. before     

B. until 

C. as         

D. since

Jack wasn't saying anything, but the teacher smiled at him________ he had done something very clever.

A．as if               

B．in case

C．while                      

D．though