–How often do you wash your hair?A. One dayB. Twice C. Twice a weekD. Two times a day

问题补充：

–How often do you wash your hair?

A. One day

B. Twice

C. Twice a week

D. Two times a day

答案：

A. One day

B. Twice

(答案→)C. Twice a week

D. Two times a day

解析：How often多久一次，提问的是频率。两次在英语中是Twice ，所以正确答案为正确C。