og sc 36为什么过去完成时？

有理想没有钱

36. Dressed as a man and using the name Robert
Shurtleff, Deborah Sampson, the fi rst woman to draw
a soldier’s pension, joined the Continental Army in
1782 at the age of 22, was injured three times, and
was discharged in 1783 because she had become too
ill to serve.
(A) 22, was injured three times, and was discharged
in 1783 because she had become
(B) 22, was injured three times, while being
discharged in 1783 because she had become
(C) 22 and was injured three times, and discharged
in 1783, being
(D) 22, injured three times, and was discharged in
1783 because she was
(E) 22, having been injured three times and
discharged in 1783, being
Parallelism; Logical predication

我知道答案选A，但是我就不觉得A里面的BECAUSE SHE HAD BECOME?????  直接SHE BECAME ......我觉得也没问题，用过去完成时是不是就是要表示病是发生在退伍之前这个概念呢，请好心人回答下，一直没弄的很明白过去完成时的用法，谢谢。。。

babybearmm

刚正好看到这篇
http://www.manhattangmat.com/forums/sc-dressed-as-a-man-and-using-the-name-robert-shurtleff-t7986.html