--do you clean the reading room?-Every two or three days.A. How longB. How farC. How so

问题补充：

--do you clean the reading room?

---Every two or three days.

A. How long

B. How far

C. How soon

D. How often

答案：

A. How long

B. How far

C. How soon

(答案→)D. How often

解析：从答句中得知，问句应为“多少时候一次”。