— do you surf the Internet ?—Once a month.A. How soonB. How oldC. How oftenD. How long

问题补充：

— do you surf the Internet ?

—Once a month.

A. How soon

B. How old

C. How often

D. How long

答案：

A. How soon

B. How old

(答案→)C. How often

D. How long

解析：根据Once a month，可知是询问频率。How soon是多久之后会发生某事；How old问年龄多大；How often多久一次，回答频率；How long多长。所以选C。