I used to (b). But, I don't kow how to explain.

-----Original Message-----
From:   Dawei Ren [SMTP:[log in to unmask]]
Sent:   Thursday, 15 July, 1999 8:21 PM
To:     [log in to unmask]
Subject:        about "ago", "before" & "previously"

   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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