--When did you become a volunteer for the elderly?-- Two years ago. I this group since the

问题补充：

--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：

A. joined

B. took part in

C. have joined

(答案→)D. have been in

解析：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。