prep 160, 关于it的指代问题

boboowu

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

解释里说，it指代前面全部，所以A选项错，我的理解it指代了 weight of discus代入到but now之后貌似没有问题啊，求NN解答

stellina

但是考虑下but now前面出现了很多单数名词诶。。it指代哪一个呢。。