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？
