On the basis of incontestable proof that car safety seats will greatly reduce the number of serious injuries sustained by children in car accidents, laws have been passed mandating the use of these seats. Unexpectedly, it has since been found that a large number of children who are riding in safety seats continue to receive serious injuries that safety seats were specifically designed to avoid, and in the prevention of which they in fact have proven to be effective. Which one of the following, if true, could by itself adequately explain the unexpected finding reported in the passage? ['The protection afforded by child safety seats depends on their being used properly, which many parents fail to do.', 'The high cost of child safety seats has caused many parents to delay purchasing them.', 'Many parents are defying the law by not using safety seats for their children.', 'Children are more likely to make automobile trips now than they were before the introduction of the safety seat.']