Uncle Tom has worked as a teacher __ he left school in 1999.a.before b.since c.after d.for

题目

Uncle Tom has worked as a teacher __ he left school in 1999.a.before b.since c.after d.for
大哥大姐帮帮忙,选什么,为什么这样选,

答案

b since引导的时间状语从句,主句用现在完成时态.