Top Law Schools

Prep19, section2, #2,
With regard to this one, I do not understand first what C means and second why E fails to be the correct answer. I did pick C as the answer but now that I reexamine it, I do not understand what C means. Thanks always.

Background: Lots of drivers violate speed limits
Premise: Exceeding speed limits is a significant factor in most accidents
Conclusion: Installing speed-limit governors would prevent most accidents

Negating choice (C), we can see that it would destroy the argument, because if exceeding the speed limit is "often" (negation of seldom) a necessary condition of avoiding an accident, it follows that "most" accidents will not be prevented.