---What should I do doctor?---healthy you should take more exercise.A. KeepB. KeepingC.

问题补充：

---What should I do, doctor?

---healthy, you should take more exercise.

A. Keep

B. Keeping

C. To keep

D. Having kept

答案：

A. Keep

B. Keeping

(答案→)C. To keep

D. Having kept

解析：句意：我应该做什么，医生。为了保持健康，你应该多做运动。动词不定式表示目的。故选C。