Sentence Correction

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.This 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

I think C should be the answer.

It brings clearly the contrast required between "once" and "now" (present) situations, without any use of redundant words like 'but' & 'and'.

you are correct. OA is C.

hitmewithgmat wrote: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.This 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

I was thinking C because of misplaced modifiers.

It's saying the weight is lined with lead around the perimeter.

It needs to say the discus is lined, not the weight of the discuss.

maybe C