A、The overweight problem. B、The car crash. C、The heart disease. D、The diabetes. B

　　(23) Car crashes are the top killer of American teenagers. Most of the crashes result from distracted driving—not paying attention to the road. Ryan Didone was a 15-year-old passenger in a car that hit a tree. He was one of the nation’s more than 30 000 victims of traffic crashes last year. Nearly 4 000 deaths, about 12%, involved drivers aged 15 to 20. His father told the story, "It was an inexperienced, immature driver who felt that he was invincible, driving at night with some kids. He was distracted. He was going too fast, and it ended up causing one death and some serious trauma and tragedy for the rest of the community. " The No. 1 cause of distracted-driving accidents is the mobile phone. Talking on the phone or reaching for it is like drinking four beers and driving. (24) Teens are especially at risk from distracted driving—and not just because they lack experience on the road, but because teens don’t really have fully developed brains until they’re 25 years old. If those two things were put together, a much, much higher risk would be got. Government and private groups are using public service announcements and events to bring more attention to this problem. (25) For example, the insurance industry recently held a safety event near Washington for teen drivers. New legislation in Congress proposes to require all states to have what is called a graduated driver licensing system. Graduated means teenagers start with restrictions like night driving and numbers of passengers. They could not get a full driver’s license until age 18.
　　23. What is the top killer of American teenagers?
　　24. Why are teens especially at risk from distracted driving according to the passage?
　　25. What do we learn about the graduated driver licensing system?