根据以下材料，回答题
Every day 25 million U.S. children fideschool buses. The safety record for these buses is much better than forpassenger cars; but nevertheless, about 10 children are killed each year ridingon large school buses, and nearly four times that number arc killed outsidebuses in the loading zones. By and large, however, the nation′s school childrenare transported to and from school safety.
Even though the number of school buscasualties (伤亡事故) is not large, the safety of children is always of intense publicconcern. While everyone wants to see children transported safely, people aredivided about what needs to be done particularly whether seat belts should bemandatory (强制).
Proponents of seat belts on schoolbuses--many of them parents and medical organizations argue that seat belts arcnecessary not only to reduce fatality and injury, but also to teach children lessonsabout the importance of using them routinely in any moving vehicle. A sidebenefit, they point out, is those seats belts help keep children in theirseats, away from the bus driver.
Opponents of seat belt installation suggestthat children are already well protected by the school buses that adhere to theNation Highway Traffic Safety Administrations (NHTSA) safety requirements setin 1977. They also believe that many children won′t wear seat belts anyway andthat may damage the belts or use them as weapons to hurt other children.
A new Research Council report on school bussafety suggests that there are alternate safety devices and procedures that maybe more effective and less expensive. For example, the study committeesuggested that raising seat backs four inches may have the same safetyeffectiveness as seat belts.
The report sponsored by the Department ofTransportation at the request of Congress, reviews seat belts extensively whiletaking a broader look at safety in and around school buses.

Each year, children killedoutside buses in the loading zones, a

A. 10

B. 40

C. 30

D. 50