The old man in Tibet for two years as a volunteer teacher；it is an experience he often mentions.
A. served
B. serves
C. is serving
D. has served
网友回答
(答案→)A. served
B. serves
C. is serving
D. has served
解析：这位老人在西藏当志愿教师两年，这已经过去了，故用过去式。句意：这位老人在西藏当志愿教师两年，这就是他经常提到的一次经历。故选A。