－ do you go to the library?
－Once a week.
A. How long
B. How soon
C. How often
D. How many times
网友回答
A. How long
B. How soon
(答案→)C. How often
D. How many times
解析：通过答语Once a week“一周一次”可以判断本句是问频率的，答案应选C。How long “多长时间”，How soon“多久”‘How many times“多少次”