This machine is very easy. Anybody can learn to use it in a few minutes. A. operatingB. to

问题补充：

This machine is very easy. Anybody can learn to use it in a few minutes.

A. operating

B. to be operating

C. operated

D. to operate

答案：

A. operating

B. to be operating

C. operated

(答案→)D. to operate

解析：表示将来，故用不定式。