Answer: The protection afforded by child safety seats depends on their being used properly, which many parents fail to do
Explanation: The expected result is that the laws mandating use of car seats proven to improve safety should cause a decrease in number of serious injuries.The unexpected result is that a large number continue to be in serious accidents.
Although Many parents oney the law by using safety seats for their children they are being used inappropriately.
(C), for the same reason, doesn't explain anything, because the result is specifically about when the seats are used, and (C) only impacts whether people will have them or not.
(D) does not explain the unexpected result, because the unexpected result has to do with injuries the seat was specifically designed to help avoid.
That leaves (B) and (E) --
(B) is super-tempting, but does not, in and of itself, help explain the unexpected result in the same way that (E) can. Even if the child is on the road more (which would of course increase # of opportunities for injury) -- it doesn't explain why the seat doesn't markedly help prevent injuries it is meant to.