单选题 .  Car crashes are the leading cause of injury and death among U. S. children, and though most of us now think of car seats as standard baby equipment, about half of all children under the age of four who died in vehicle accidents last year were no restrained. It is calculated that only about two-thirds of children aged five to fifteen buckle their seat belts.
    Moreover, the traffic-safety agency estimates that even among parents who always strap their children in, 85% are not doing it properly. They often don't know where best to place the kids, don't use the proper restraint for their age and weight, or don't install the safety seats properly. Despite the reports about front seats collapsing onto back seats when certain car models get in accidents, the safest place in the car for any child up to the age of 12 is still the back seat. Babies up to 9 kg and one year old should ride in rear-facing infant seats.
    Never place a child under age 12 in the front seat with a working passenger-side air bag. These devices are discharged at 320 km/h and can be triggered by low-speed fender benders. They have killed 77 kids in the U. S. since 1993. If you must place a child in front, make sure the passenger-side bag is switched off.
    Children over age one should ride in forward-facing safety seats with a five-point harness system. A child who weighs at least 18 kg or at least I m high can graduate to a booster seat that elevates her so that the standard shoulder and lap belt fits properly.21.  What does the author mainly discuss in this passage? ______