OG12-23求助

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
答案选a. 我想知道为什么it在这里不能指代weight of the discus？