[SC资料] 请教一道分句修饰主语的题【铁饼设计】

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

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

B.     According to a previous design, the weight of the discus used in trackcompetition was concentrated in a metal center, but now it is

C.     Once designed with its weight concentrated in a metal center, the discusused in track competition is now

D.     The discus used in trackcompetition, once designed with its weight concentrated in a metal center, butnow         (C)

E.     The discus used in track competition was once designed having its weightconcentrated in a metal center and now

这道题OA是C，想请问一下各位大牛，【Once designed with its weight concentrated in a metal center】应该修饰后面句子的主语【the discus】，但是the discus不会自己design自己啊？难道不应该是Once 【been】 designed with its weight concentrated in a metal center才对么？看的时候第一眼就把C排除掉了，求解答，感谢感谢！