There is such a choice question as below:
A german taxi-driver, Franz Bussman, recently found a brother who was thought to have been killed twenty years  _____.
there are three choices: (a) ago (b)before (c) previously
I think (a) is right, (b) is also right, (c) is not right.  But the answer on the book is (b) or (c). Why? Who could explain it to me? Thanks.

Dawei Ren
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