did you decide to be a teacher?

题目

did you decide to be a teacher?
Let me see.I am thirsty,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

答案

since 后面只能加 “点时间”
a错在这是一段持续的时间,如果选a就译为：自从二十年 明显不通顺
b错在这个短语只能单独用 意思就是如果要用就不能和since一起用
c和a犯的是同一个错误
所以用排除法也能选出d