—John do you visit your grandparents?—Once a week.A. how longB. how soonC. how oftenD. h

问题补充：

—John, do you visit your grandparents?

—Once a week.

A. how long

B. how soon

C. how often

D. how much

答案：

A. how long

B. how soon

(答案→)C. how often

D. how much

解析：how long多长；how soon多久；how often多久一次；how much多少钱。根据下文，每周一次。可知上文提问频率，故选C。