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.

I think your book is right.
"Ago" should be used with the simple past while "before" with the past
perfect
d. I saw that film two days ago.
e. *I saw that film two days before.
f. *I had seen that film two days ago.
g. I had seen that film two days before.
Of course, "before" alone can be used with the past and other tenses:
h. I saw that film before.
i. I have seen that film before.
j. I had seen that film before.

By the way, "who was thought to have been killed ..." can be paraphrased
as
k. who they thought had been killed ...
so that from a grammatical point of view (b) is right but (a) is not.

As for "previously" I have an example which is used with the past perfect.
l. Several years previously I had seen four Faberge eggs exhibited at a
Manhattan art gallery. -- Lawrence Sanders, McNally's Gamble 1997:19

Can this be of any help?

Not an English native speaker SIRAISI Tomio
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