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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    Did you know that your small intestine is nearly six meters long? Or that there are about 60 muscles in your face, and you use 40 of them to frown but only 20 to smile? How about the fact that our bodies consist of 73 percent water, and that our hearts beat over 100,000 times each day? You really are amazing!    The human body is a complex machine. From the day we are born, our bodies grow and change in response to our environment, diet, and habits. The body has many different organ systems and parts that work together to allow us to breathe, move, see, talk, and digest food all at the same time. Most of the time we are unaware of what is happening in our bodies, usually it is only when we get sick or feel pain that we notice.    Many people do not take care of their complex machines. Bad habits like smoking, drinking too much alcohol, and eating junk food damage our bodies. Stress can also cause health problems. People who worry a lot or have busy jobs often don't get enough sleep, or don't eat properly. We also can damage our bodies when we play sports or get into accidents. Studies done by the Australian government show that most people get hurt because of an accidental slip or fall, or because of injuries from car accidents. It's true that a lot of people go to hospital because of serious illnesses, but far more people end up there because they simply weren't being carefill.    Like machines, different body parts sometimes wear down from old age. People over the age of 65 are more likely to fall and hurt themselves. And these injuries - from bad cuts to broken bones - usually require serious medical attention. Due to the increase in the population of elderly people, gerontology is now one of the fastest growing areas of medicine. There are many treatments available to help older people recover from illness and injury. It is now common for older people with damaged joints, for example, to have surgery to replace the old joint with a new one made of plastic or metal. Instead of suffering aches and pains through their retirement days, older people are able to lead happier and more comfortable lives.    As with any machine, the better you take care of it, the longer it will last. The best way to take care of your amazing machine is to eat the right foods, do regular exercise, and get enough sleep. Oh, and don't forget to smile!    Our body is compared to a machine because __________.