A. has begun; time　　　　　　　　　　　　　　  B. has been on; a time

C. has been on; time　　　　　　　　　　　　   D. has begun; a time

The teacher asked a difficult question, but finally Tom came ________ with a good answer.

It is not difficult for us to find out that all the events ________ each other closely.

Anyone who wants to be a success should ________ talent ________ effort.

The weather is not very pleasant, but ________ the fog has gone.

The great scientist has made ________ important discoveries already.

Don’t ask questions ________. One by one, please.

We must ________ the possibility that the train may be late.