In many sports, physical contact  is part of the game. But when athletes hit their heads, the hit can cause concussions . Concussions have long been a concern for professional athletes, but they've become more common among young players too. Between 2001 and 2009, the number of kids under 19 visiting doctors for concussions each year increased by 62 percent. Engineers are developing new helmets , and new rules are limiting physical contact in some sports. But are these efforts enough to protect kids' brains? Concussions can happen during many types of activities. But sports like football and soccer are especially risky. When you run or jump, your body is moving at an increased speed. A sudden hit causes you to move faster in a different direction. "It's like when you're in a bus and it makes a sudden stop," says Dr. Kevin Crutchfield, an expert at a sports concussion center in Baltimore, Maryland. "You hit the inside of the bus." This hit damages the brain. It affects the way the brain functions. So some sports require athletes to wear helmets. Helmets help reduce the force of a hit. The hard outer shell spreads the force of larger area. Inside a helmet, a plastic or air-filled lining also helps soften the blow. Steve Rowson is an engineer at Virginia Tech. In his lab, he tests how helmets hold up against different types of hits. No helmet can prevent concussions completely, says Rowson. But he's found that when wearing a newer football helmet, the player's head doesn't gain speed as much after a hit. That lowers the risk of concussions. Many youth sports teams have set limits on physical contact to reduce the risk of concussions. In soccer, most concussions happen as a result of headers--shots or passes made with the head. That's why many people think kids shouldn't head the ball until high school. These efforts should help, says Crutchfield. But the best way to prevent long-term brain injuries, he says, is to stop playing immediately if you suffer concussion. "Protect yourself today so you can play again tomorrow." The author mentions the sudden stop of the bus to   _  .
stress it's dangerous to ride buses show human brains are easily damaged give a better explanation of concussions remind people to wear helmets while playing sports
Question: In many sports, physical contact  is part of the game. But when athletes hit their heads, the hit can cause concussions . Concussions have long been a concern for professional athletes, but they've become more common among young players too. Between 2001 and 2009, the number of kids under 19 visiting doctors for concussions each year increased by 62 percent. Engineers are developing new helmets , and new rules are limiting physical contact in some sports. But are these efforts enough to protect kids' brains? Concussions can happen during many types of activities. But sports like football and soccer are especially risky. When you run or jump, your body is moving at an increased speed. A sudden hit causes you to move faster in a different direction. "It's like when you're in a bus and it makes a sudden stop," says Dr. Kevin Crutchfield, an expert at a sports concussion center in Baltimore, Maryland. "You hit the inside of the bus." This hit damages the brain. It affects the way the brain functions. So some sports require athletes to wear helmets. Helmets help reduce the force of a hit. The hard outer shell spreads the force of larger area. Inside a helmet, a plastic or air-filled lining also helps soften the blow. Steve Rowson is an engineer at Virginia Tech. In his lab, he tests how helmets hold up against different types of hits. No helmet can prevent concussions completely, says Rowson. But he's found that when wearing a newer football helmet, the player's head doesn't gain speed as much after a hit. That lowers the risk of concussions. Many youth sports teams have set limits on physical contact to reduce the risk of concussions. In soccer, most concussions happen as a result of headers--shots or passes made with the head. That's why many people think kids shouldn't head the ball until high school. These efforts should help, says Crutchfield. But the best way to prevent long-term brain injuries, he says, is to stop playing immediately if you suffer concussion. "Protect yourself today so you can play again tomorrow." The author mentions the sudden stop of the bus to   _  .
A. stress it's dangerous to ride buses
B. show human brains are easily damaged
C. give a better explanation of concussions
D. remind people to wear helmets while playing sports
Answer: