I believe here is somebody _________ on the door.

A. to knock B. knock C. knocked D. knocking

She swam to the shore _________ the night in the water.

A. having spent B. having spending C. when spending D. had spent

When _________ in international trade, letters of credit are convenient.

A. used B. are used C. using D. they used

After _________ dinner, I watch television

A. eat B. eating C. eaten D. ate

_________ by the visitor, the clavichord could not be used.

A. Broken B. Break C. Breaking D. Broke

________ their farm work, the farmers returned home.

A. Finishing B. Finish C. Having finished D. Being finished

The girl_____ behind you is naughty.

A. stands B. stood C. is standing D. standing

_________ their work, they went home.

A. Finishing B. Having finished C. Had finished D. Had finished

By the time Robert will finish writing the first draft of his paper, most of the other students will have completed their final draft.

A. will finish

B. writing 

C. most 

D. their 

What we know about certain diseases are still not sufficient to prevent them from spreading easily among the population.

A. What we know

B. are

C. from spreading

D. among