GMAT PREP (SC)

C

A - Because it says "in a previous design," "is" should be "was" because it is discussing a past form.

B - According to a previous design doesn't work because that is how it WAS previously designed. (Don't know if this explanation makes sense)

D - There is no verb for the main clause, thus making it a fragment.

E - "and now" doesn't work because there is a difference between the current design and the previous design. The conjunction "and" does not show that there is a change or difference.

The major issue with the original question is the usage of the pronoun "it." The intended meaning of the sentence is that the *discus* is now lined with lead. As the sentence is originally written, "it" refers to the *weight" of the discus (there are several singular nouns used in the sentence before "it"--design, weight, competition, center--but these other words are all placed in modifiers...IN a previous design, OF a discus, IN a metal center. )

The intended antecedent of "it" is reinforced by parallel structure of Subject-Verb, Subject-Verb in the two clauses: the discus IS USED...but now IS LINED. Choice B suffers from this same error, so you're down to C, D, and E.

Choice C corrects the original error by losing "it" as a subject altogether and replacing it with "discus."

Span is dead on when he says that choice D is a fragment-- no verb, no sentence! If the word "but" was replace with the word "is," the sentence would be absolutely fine. Notice that choice D actually makes what is a far more intuitive correction for most students--placing the subject "discus" at the beginning of the sentence...only to take a wrong turn at the 2nd to last word. The GMAT is trying to trap lazy readers, so be vigilant! Choice E not only uses "and" where the meaning demands "but" (rock on yet again Span!), but also contains the awkward "having" instead of "with."

my pick is c. just keep the modifier as close as you can to the main subject (in this case is the discus). your clue is the final unmarked part of the sentence.