We are having a class get-together. It’s been twenty years we graduated from high school.

问题补充：

We are having a class get-together. It’s been twenty years we graduated from high school.

A. before

B. since

C. after

D. when

答案：

A. before

(答案→)B. since

C. after

D. when

解析：考查句型：It is/has been+时间段+since后用一般过去式，自从什么时候起已有一段时间了。故选B。