问题补充：
-- did your uncle leave his hometown ?
--He for nearly twenty years.
A. When, has left
B. When, has been away
C. How long, has left
D. How long, has been away
答案：
A. When, has left
(答案→)B. When, has been away
C. How long, has left
D. How long, has been away
解析：根据答语内容可知问的内容可知答案为B，for +一段时间，用have been aways，表示，故选B
