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

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：How often 多久一次；How long多长；How old 多大；How soon多久。根据下文答语，大约三天。可知上文提问时间长短，故选B。