I have the sentence from 合格できる、日本語能力試験N2:
私は外国旅行をするたび、「 」専門の勉強をしている日本人の若い学者に会うと、思いがけない収穫をえる。
The choices for the blank are A)あそこで B)そこで C)ここで D)どこかで
I narrowed it down to either A or B, but then I realized that I don't know the difference between them. Would someone mind explaining when either should be used and the difference they make?