择题When did you decide to be a teacher? Let me see.I am thirty,and I have wanted to be one since_a twenty years b twenty years ago c I had twenty years d I was twenty答案为什么是dsince+一段时间+ago为什么不可以？
问题描述：
择题When did you decide to be a teacher? Let me see.I am thirty,and I have wanted to be one since_
a twenty years b twenty years ago c I had twenty years d I was twenty
答案为什么是d
since+一段时间+ago为什么不可以？
答