-- do you go to the cinema?-- Once a week.A. How soonB. How longC. How farD. How often

问题补充：

-- do you go to the cinema?

-- Once a week.

A. How soon

B. How long

C. How far

D. How often

答案：

A. How soon

B. How long

C. How far

(答案→)D. How often

解析：根据答语，可知：答案how often表示多久一次，符合题意。