问题
选择题
--When did you become a volunteer for the elderly?
-- Two years ago. I this group since then.
A．joined
B．took part in
C．have joined
D．have been in
答案
答案：D
有since短语或从句修饰主语通常用完成时，结构为have+过去分词，但在完成时
短暂动词不能和时间状语连用，join属于短暂动词，故只能选D。