请教一道语法题

In a previous design, the weight of the discus used in track competition is concentrated in a metal center, but now it is lined with lead around the perimeter, thereby improving stability in flight an resulting in longer throws.

(A)    In a previous design, the weight of the discus used in track competition is concentrated in a metal center, but now it is

(B)    According to a previous design, the weight of the discus used in track competition was concentrated in a metal center, but now it is

(C)    Once designed with its weight concentrated in a metal center, the discus used in track competition is now

(D)    The discus used in track competition, once designed with its weight concentrated in a metal center, but now

(E)     The discus used in track competition was once designed having its weight concentrated in a metal center and now

答案是C，B怎么错了？

浅见如下：选B的话主句主语就成了The weight了,后面的逻辑就不对了。

答案是E吗