23. 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 and 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

为什么说b选项里的it指代的是discus，所以b不对呢？it不能指代weight么?...