since 和so far

题目

since 和so far
：§ I have been working in this school since my graduation .§ Oh ,I see .You have taught nowhere else ( A ).A.since B.so far C.ago D.already 为什么不是B ,怎么是A

答案

so far 到目前为止
since 自XX开始
ago XX以前
already 已经
从题意上看是,我一直在这所学校工作自从我毕业以后.所以应该选择 A since