Vorland adopted new regulations requiring airbags and better seat belts in all new automobiles 10 yrs ago. (regulations that make a car safer in case of an accident)
Since then, the annual number of drivers and passengers hospitalized overnight or longer for automobile accidents has steadily declined. (So serious injuries have gone down.)

Clearly, the regulations have made Vorland's automobiles safer for drivers and passengers who are involved in accidents.

The argument is trying to tell us that of 100 accidents, if serious injuries used to be 50 before, now they are only 25 because of better seat belts and air bags. So from 50 serious injuries, now we have only 25.

We need an assumption.

(A) The citizens of Vorland generally obey all national and local speed limits.

Irrelevant. Something that has stayed the same has no impact on our argument.

(B) Of the patients treated at hospitals in Vorland, the percentage treated being treated for injuries sustained in automobile accidents has decreased in the past ten years.

Percentage of accident treatments is irrelevant. We don't even know whether number of auto accident injuries treated has increased or decreased. It depends on the total number of patients treated.

(C) The safety improvements required by the regulations have not significantly affect the price of new automobiles.

Price of new automobiles is not very relevant directly. If the prices have increased significantly and hence number of vehicles has reduced, that could imply fewer accidents and hence fewer serious injuries. But it sure is a long shot. Let's move ahead.

(D) There has been no increase in speed limits on Vorland's major highways over the past ten years.

Increase in speed limit is irrelevant. Whether people actually drove at higher/lower speeds is not known.

(E) Over the past ten years, there has not been a steady decline in the annual number of drivers and passengers involved in automobile accidents in Vorland.

Correct. This is our assumption. If we are to say that the cars have become safer for people involved in accidents, we are assuming that total number of accidents has not reduced. Perhaps people have started driving more slowly and number of accidents has reduced. Perhaps from 100 accidents, now only 50 happen. So number of serious injuries would have automatically gone down. We are assuming that this did not happen. That actually 100 accidents took place as usual and only 25 serious injuries took place. So the new regulations did make the car safer for those involved in accidents.

Let's negate (E)
(E) There has been a steady decline in the annual number of drivers and passengers involved in automobile accidents in Vorland.
If number of accidents has been decreasing, it makes sense that serious injuries would decrease too. Then we can't say that the new regulations have made the cars safer for people involved in accidents.

Answer (E)