To: Davei Ren

To me all three are idiomatic, although (a) has a somewhat different meaning from (b) and (c).  "Twenty years ago" means twenty years prior to the
time the sentence is spoken.  "Twenty years before" (or "previously") means twenty years prior to the time described in the sentence, which may be
different.  For example, if I were to say today, "In 1990, I met the person who won the Nobel Prize twenty years ago," it means the person won the
prize in 1979.  If I were to say "In 1990, I met the person who won the Nobel Prize twenty years previously," it means the person won the prize in
1970.

Dick Veit

Dawei Ren wrote:

>    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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