.--are they staying in Beijing?----For three days. A. How often B. How longC. How oldD.

问题补充：

. ----are they staying in Beijing?

----For three days.

A. How often

B. How long

C. How old

D. How soon

答案：

A. How often

(答案→)B. How long

C. How old

D. How soon

解析：for three days表示的是时间段，故用how long 来提问。