do you go to see your grandparents?Once a week.A. WhenB. How much C. How oftenD.

问题补充：

--- do you go to see your grandparents?

--- Once a week.

A. When

B. How much

C. How often

D. How long

答案：

A. When

B. How much

(答案→)C. How often

D. How long

解析：When什么时候；How much多少钱；How often多久一次；How long多长。根据答语，每周一次。可知上文对频度进行提问，选C。