单选题 .  What is in the drug that makes you sick or dead?     41    , cocaine is harmful, but what makes it harmful?
    I can't tell you about drugs, but I can help you think about them in this simple way. Your body is a very     42    machine, really a lot of chemical machinery, all of which     43    so that it all works together. Special     44    , which we call drugs, can affect it or     45    it in many different ways.
    Some drugs are     46    when your body has a problem,     47    disease-causing bacteria. Then someone may give you aspirin to keep your temperature from going too     48    or some penicillin because it stops the growth of some kinds of bacteria.     49    , all drugs are really poison,     50    if you take too much, so you must always use them carefully.
    Why do some people take drugs like cocaine? For a little while they seem to make you feel     51    , or smarter, or happier. But after their effects have worn     52    , your body has to pay an extra     53    to get back to     54    . That makes you want to get some more of the drug. So we say they are addictive.
    Drugs like cocaine have their special effects     55    they act as special poisons for your nervous system. They disconnect some of your nerve pathways and take     56    some of your senses and your worries. They make you want always more. And just a little     57    much can even block nerves to your heart and stop its beating.
    Many of us worry about the environment around us and what pollution does to     58    How about your internal environment, and what     59    inside you? You control that all by yourself in what you put into your     60    . Most drugs are pollutants. You would not want pollutants in the air and water around you. Why would you want pollutants in your body?41.