For this question, it helps to paraphrase the discrepancy.
There are two scenarios. In some areas, only certain drivers use headlights in good visibility, and they avoid collisions more often than people who don't use the headlights. In other areas, everyone uses headlights. If the headlights helped, you would expect fewer collisions overall -- but that doesn't happen. So, if it the headlights don't help, why are those drivers in the first areas getting into fewer accidents?
(C) offers an explanation. It's not the headlights, but the drivers themselves.
(E) only discusses the visibility in the jurisdictions that mandate using headlights. Without knowing what visibility is like in the other jurisdictions, this is an incomplete comparison and therefore doesn't help.