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
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发表于 2010-12-10 12:54:26
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