LSAT 97-02-1-14

In jurisdictions where use of headlights is optional when visibility is good, drivers who use headlights at all times are less likely to be involved in a collision than are drivers whose use headlights only when visibility is poor. Yet Hightway Safety Department records show that making use of headlights mandatory at all times does nothing to reduce the overall number of collisions.

which one of the following, if true, most helps to resolve the apparent discrepancy in the information above?

(C)only very careful drivers use headlights when their use is not legally required.

this is the correct answer?   Can someone explain why ? Thanks!

没有法律规定的时候,只有careful driver开灯.即事故降低的原因都归于这些人.

有法律规定的时候, 除了careful driver还加上 uncareful driver开灯.但是后者并不对事故降低有影响.所以有没有法律规定,效果是一样的.