题目内容：
Passage 2Questions are based on the following passage．
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 nojoke．(77)Teenagers who don't get enough rest have more learning,health,behavior and mood problems man students who get at least Iline 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 highschool students，University of Colorado researchers found that 82 percent said they woke up tiredand 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 messages to friends often keep them awake deep into the night．Taking soda and energy drinks late in the day and going to latenight parties on weekends add to sleep(1ebt．Biology also works against teenagers’sleep．The 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 l l pm．Class usually begins before 8：15am，with many high schools starting as early as 7：15 am．To get to school on time，most teenshave to get up by 6：30 al／l，guaranteeing they’11 be sleep—deprived during the week．Teens oftensleep much later on weekends to catch up，making it even harder to fall asleep on Sunday night andwake up Monday mornin9．Playing catch—up on weekends also doesn’t help teens stay refreshed
when they need it most：during the week at school．
Since the l990s，middle and high schools in more than two dozen states have experimented with later school start times．(78)The results have been encouraging：more sleep,increased at tendance,better grades and fewer driving accidents．But most schools still start early，meaning teens have their work cut out for them if they want to get enough sleep．
According to the passage，poor sleep can be linked to all of the following EXCEPT_________． A．heart disease
B．cai accidents
C．skin problems
D．poor concentration
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