Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions 

     A recent study suggests that teens' relationship with parents, friends and teachers may have a lot to do with why they don't get a good night's sleep. 

     David Maume, a sociologist and sleep researcher, analyzed federal health data, which interviewed 974 teenagers when they were 12, and then again at 15. He found that family dynamics have much to do with how well kids sleep. Teens, who had warm relations with their parents and felt like they could talk to them or their parents were supportive of them, tended to sleep better. However, families that were going through a divorce or a remarriage tended to affect teens' sleep. 

     And problems at school also affected teens' sleep. Feeling safe at school and having good relations with teachers tended to promote better sleep. As did good relationships with friends. Kids who took part in sports or other positive social activities or shared similar academic goals with their friends were also more likely to get a good night's sleep. 

     These add up to what makes lots of sense: a general feeling of well-being helps teens sleep. If we're happy and contented, we're much more likely to sleep better than if we're sad and anxious. Now, of course, teens can hardly resist being drawn to their computers and social networking. Maume also found that when parents were strict not only about bedtime, but also about limiting technology, kids slept better. It's a finding that seems obvious, but parents really do matter when it comes to health habits of their teenagers. Clearly, teenagers aren't getting 9 to 10 hours a night, which puts them at risk for all the consequences of lack of sleep, including poor academic performance, colds and stress.

(Adapted from www.mofangge.com/html/qDetail/03/93/.../61359303219584.html) 

What does the word "them" in the passage refer to?