The field research will take Joan and Paul about five months;it will be a long timewe meet

问题补充：

The field research will take Joan and Paul about five months;it will be a long timewe meet them again.

A. after

B. before

C. since

D. when

答案：

A. after

(答案→)B. before

C. since

D. when

解析：before意为“在……之前”。根据所提供的情景The field research will take Joan and Paul about five months可判断出表示“要很长时间我们才能再见面”。after意为“在……之后”。since引导时间状语从句意为“自从”，表示动作从过去的某个时候开始一直延续到现在，常与现在完成时连用。when意为“当……的时候”。