—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：我能借这本书多久? how long ,多久，用来提问一段时间。BD是频率，A不完整，故选C。