答案　B　[根据句意可以看出这里的代词是指代前面的frontiers，因此应该用them。]

　　Ⅴ.阅读理解

　　One of our worst nightmares as a parent is finding out that our teenager has started smoking or taking drugs.Usually our first reaction is to be angry，but most of us can remember what it was like when we were at school and just how powerful peer(同龄人)pressure can be.Once we are faced with such a case，we ought to be understanding and supportive and should try to help our children through the problems.

　　Many teenagers are attracted to drugs at some stage in their life and research shows that as many as 15% of all teenagers have tried drugtaking of some sort during their young lives.Besides the usual peer pressure and the constant need to be“cool”or to have“fun”，many teenagers are simply curious about the excitement or“high”they can achieve from taking drugs.That excitement is doubled by the simple fact that they are doing something illegal that we as parents would consider dangerous.

　　On a more serious note，they might be getting into drugs as an escape.Perhaps they are unhappy at school or there have been recent changes at home that may have unsettled them.In this case，drug taking can be more worrying as they may start to rely on it as a form of escapism and start to take drugs more regularly.

　　If you think your child might be smoking or taking drugs，it's best to deal with it straight away.It's easy to turn a blind eye and sweep_the_issues_under_the_carpet in the hope that they'll gradually stop，but without your intervention(干预)，the problem may not disappear.

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