C. Sleeping                          D. Having slept

(20) ______ the flies out, we had to shut all the doors and windows.

A. Keep                              B. To keep

C. Keeping                          D. Having kept

(21) ______ their service, the workers of the hotel are active in learning English.

A. Improving                       B. To improve

C. Improve                          D. Having improved

 

 

 

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