do you have piano lessons?Once a week.A. How manyB. How longC. How oftenD. How so

问题补充：

--- do you have piano lessons?

--- Once a week.

A. How many

B. How long

C. How often

D. How soon

答案：

A. How many

B. How long

(答案→)C. How often

D. How soon

解析：本题的回答语表示的是频率，对此提问用how often，故本题选C。