When I went back to my home town three years ago, I found that a lot of changes ____.

Question

(A) are taken place
(B) were taken place
(C) have taken place
(D) had taken place
I answered it as B. but in the book the correct answer was D. please explain by giving examples.

Answer 1

http://www.englishpage.com/verbpage/pastperfect.html

It's a part of the sequence of tenses. Because the changes took place before the main verb ("went"), past perfect ("had taken place") is needed.