The company has been producing this model of machine tool ________ 2008.A.since B.for C.before
问题描述：
The company has been producing this model of machine tool ________ 2008.A.since B.for C.before
答案选A为什么不选before.before作为副词不是也能引导现在完成时和过去完成时么.
答
before 是介词,在……之前,时间已过去,不能与现在完成时连用,而SINCE 后接过去的时间点（2008）,常与一般现在时连用