—do you go to the library? —Once a week.A. How many B. How often C. How long

问题补充：

—do you go to the library?

—Once a week.

A. How many B. How often C. How long

答案：

解析：句意为：你多久去一次图书馆？---每周一次。How long多长，How many多少，How often多久一次。根据句意可知应选B。