单项选择题Every day 25 milion U. S. children ride school buses. The safety record for these buses is much better than for passenger cars; but nevertheless, about 10 children are killed each year riding on large school buses, and nearly four times that number are killed outside buses in the loading zones. By and large, however, the nation's school children are transported to and from school safely. Even though the number of school bus casualties(死亡人数) is not large, the safety of children is always of intense public concern. While everyone wants to see children transported safely, people are divided about what needs to be done—particularly whether seat belts should be mandatory (强制性的). Supporters of seat belts on school buses argue that seat belts are necessary not only to reduce death and injury, but also to teach children lessons about the importance of using them routinely in any moving vehicle. A side benefit, they point out, is that seat belts help keep children in their seats, away from the bus driver. Opponents of seat belt installation suggest that children are already well protected by the school buses that follow the National Highway Traffic Safety Administration's (NHTSA) safety requirement set in 1977. They also believe that many children won't wear seat belts anyway, and that they may damage the belts or use them as weapons to hurt other children. A new Research Council report on school bus safety suggests that there are alternate safety devices and procedures that may be more effective and less expensive. For example, the study committee suggested that raising seat backs four inches may have the same safety effectiveness as seat belts. The report sponsored by the Department of Transportation at the request of Congress, reviews seat belts extensively while taking a broader look at safety in and around school buses. Each year, children killed outside buses in the loading zones are about_______.
A.10
B.40
C.30
D.50
