160.
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 and 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C，谁来讲解下这道题？
排除AB是不是因为once和后面的now对应？
E错哪了？是不是was once designed中间加个once就不能表被动了？ | 
