问题补充:
--- have you been a member of Greener China?
--- For two years.
A. How often
B. How long
C. When
D. How far
答案:
A. How often
(答案→)B. How long
C. When
D. How far
解析:How often多久一次;How long多长,多久;When什么时候;How far多远;根据下文,两年了。可知上文指的是,你成为绿色中国的一员多久了?故选B。