--When did you bee 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
网友回答
A. joined
B. took part in
C. have joined
(答案→)D. have been in
解析：由“since then”可知，此句时态为现在完成时，故排除AB，又因为join为非延续性动词不可与现在完成时连用，排除C，故选D