单选题 --When did you become a volunteer?
--Two years ago. I ______ this group since then.
A. have joined B. joined to C. have been in

【正确答案】 C

【答案解析】句意：“你什么时候成为志愿者的?”“两年前。从那以后我就加入这个组了。”本题考查时态和动词的用法。本题中根据时间状语since then判断时态为现在完成时，必须与延续性动词连用。havejoined为瞬间动词，have been in表示状态有延续性，故选C。