—How long can I keep the books?—A. Three. B. Three times.C. Three days.D. Very often.

问题补充：

—How long can I keep the books?

—

A. Three.

B. Three times.

C. Three days.

D. Very often.

答案：

A. Three.

B. Three times.

(答案→)C. Three days.

D. Very often.

解析：句意:--我可以借这本书多长时间了？--三天。how long用来提问一段时间，而选项A不完整，选项B、D均表示频度，故选C。