That image may make you laugh, but lack of sleep is no joke. (77) Teenagers who don’t get enough rest have more learning, health, behavior and mood problems than students who get at least nine hours a night. Long-term lack of sleep is tied to heart disease, overweight, depression and a shortened life span in adults, indicating the importance of establishing good sleep habits early in life. Lack of sleep can be especially deadly for teens; car accidents are the leading cause of death among teenagers, and safety experts believe sleepy driving is a major factor. Unfortunately, few teenagers get the sleep they need. In a survey of middle-school and high-school students, University of Colorado researchers found that 82 percent said they woke up tied and unrefreshed, and more than half had trouble concentrating during the day at least once a week. Blame multitasking for some of this. Many students are juggling after-school activities, homework and part-time jobs. Even when they manage to fulfill these obligations by a reasonable hour, television, the internet, video games, phone calls and text message to friends often keep them awake deep into the night. Taking soda and energy drinks late in the day and going to late-night parties on weekends add to sleep debt. Biology also works against teenagers’ sleep. Their body’s internal clock, which controls when a person starts to feel tired, shifts after puberty (青春期), making it hard for most teens to fall asleep before 11 pm. Class usually beings before 8:15 am., with many high schools starting at 7:15 am. To get to school on time, most teens have to get up by 6:30 am, guaranteeing they’ll be sleep-deprived during the week. Teens often sleep much later on weekends to catch up, making it even harder to fall asleep on Sunday night and wake up Monday morning. Playing catch-up on weekends also doesn’t help teens stay refreshed when they need it most: during the week at school. Since the 1990s, middle and high school in more than two dozen states have experimented with later school start times. (78) The results have been encouraging: more sleep, increased attendance, better grades and fewer driving accidents. But most school still start early, meaning teens have their work cut out for them if they want to get enough sleep.The main idea of Paragraph 3 is ______. A．how sleep deprivation (缺乏) can be treatedB．what causes sleep deprivationC．who is most at risk for sleep deprivationD．why sleep deprivation is a serious concern

As the school year kicks off, parents are once again struggling to cajole (哄骗) and, if need be, drag their exhausted teens out of bed. That image may make you laugh, but lack of sleep is no joke. (77) Teenagers who don’t get enough rest have more learning, health, behavior and mood problems than students who get at least nine hours a night. Long-term lack of sleep is tied to heart disease, overweight, depression and a shortened life span in adults, indicating the importance of establishing good sleep habits early in life. Lack of sleep can be especially deadly for teens; car accidents are the leading cause of death among teenagers, and safety experts believe sleepy driving is a major factor. Unfortunately, few teenagers get the sleep they need. In a survey of middle-school and high-school students, University of Colorado researchers found that 82 percent said they woke up tied and unrefreshed, and more than half had trouble concentrating during the day at least once a week. Blame multitasking for some of this. Many students are juggling after-school activities, homework and part-time jobs. Even when they manage to fulfill these obligations by a reasonable hour, television, the internet, video games, phone calls and text message to friends often keep them awake deep into the night. Taking soda and energy drinks late in the day and going to late-night parties on weekends add to sleep debt. Biology also works against teenagers’ sleep. Their body’s internal clock, which controls when a person starts to feel tired, shifts after puberty (青春期), making it hard for most teens to fall asleep before 11 pm. Class usually beings before 8:15 am., with many high schools starting at 7:15 am. To get to school on time, most teens have to get up by 6:30 am, guaranteeing they’ll be sleep-deprived during the week. Teens often sleep much later on weekends to catch up, making it even harder to fall asleep on Sunday night and wake up Monday morning. Playing catch-up on weekends also doesn’t help teens stay refreshed when they need it most: during the week at school. Since the 1990s, middle and high school in more than two dozen states have experimented with later school start times. (78) The results have been encouraging: more sleep, increased attendance, better grades and fewer driving accidents. But most school still start early, meaning teens have their work cut out for them if they want to get enough sleep.The main idea of Paragraph 3 is ______. A．how sleep deprivation (缺乏) can be treatedB．what causes sleep deprivationC．who is most at risk for sleep deprivationD．why sleep deprivation is a serious concern