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    There is a lake near a forest in Sweden. The water in the lake is very clear and everything is quiet and still. On the far side of the water is some farmland and further away, across the field, is an old church built on a hill. It has just stopped raining. The brook(小溪) flowing into the lake is full and there is still some half-melted(半融化的) snow from last winter on the ground.

    The reason the water is so clear, and everything is so quiet and still, is that most of the plant and animal life in and around the lake is dead. There are no birds because all the fish and insects, which the birds need for food, have disappeared. Many trees in the forest are losing their leaves and bark (树皮), and are sick and dying. Others were blown down by the high winds of last winter. A farmer is working in a field planting something, but unless he plants a stronger kind of seed this year, the crop will be between 25 to 40 percent lower than a few years ago. The stones of the old church tower are turning to powder and unless the tower is repaired soon, it will fall down.

    All the death and destruction(破坏) in this place is caused by the smoke coming out of factories and poisonous gases from cars and trucks. And yet this silent lake is hundreds of meters aways from the nearest factory and highway.

    Big data (数据) is a lot of sets of information that are put together so they can be used by a computer program. The computer program looks for different kinds of answers or patterns in the data. Big data can have different kinds of information from many sources, such as information that comes from schools, social media sites, companies, and governments. One set of data can have people's names and addresses. Another set can have what they like, where they go to school, and how much time they spend on the computer.

    Big data can be used in many ways. The government uses it to understand how many people travel on buses or trains. This information is then used to make bus or train systems better. Some schools use big data to understand which children may need extra help in class. The teacher can then give certain students more help or support so those students can succeed in school. Companies use big data, too. It helps them understand who buys their products. For example, one company uses weather data to see when people eat the most ice cream.

    Big data can be used for good reasons. Some hospitals use big data to predict if a baby who is born too early will get sick. The hospital can then take extra steps to take care of that baby so he or she does not get sick. Big data can also be used for bad reasons. It can be used to predict which kinds of people are likely to break the law or hurt others, even if they have not done anything wrong.

    How can big data be used for good and bad things? It is because the computer programs used to look at big data and understand it are written by people. People think a certain way. Because they think a certain way, they build a model of those ideas. These ideas are then used to look at the data. Sometimes, these ideas are helpful for people or businesses. Other times, bad ideas can cause problems for certain groups of people.

    It is a cold and dry winter morning. When you wake up, you feel that your throat hurts. You begin to sneeze and later you may cough. You must have a cold！Wait a moment. Could it just be a common cold or the flu(流感)that's been going around？

    A cold gives you a runny nose and cough. But it's usually easy to deal with. Some people take medicine while others drink herbal tea or chicken soup. Usually you feel better after a week.

    On the other hand, the flu can be much more dangerous. It usually gives you fever. Sometimes it can cause death. According to an American magazine, the worst flu killed about 50 million people from 1918 to 1919.

    Don't worry. If you get a flu vaccine(疫苗), it can stop you from getting the virus(病毒).Flu vaccines work by giving the body a small number of flu viruses. This causes your body to develop antibodies(抗体). So next time you get the flu, the antibodies will help you.

    The flu virus changes every few years. Therefore, there are so many types of flu, such as HlN1 and H5N1.The antibodies for one type of flu can't help us against all types of flu. That's why we need to get the vaccine every year.

    However, good healthy habits may also help stop you developing a cold or flu. Washing your hands often helps stop you from getting viruses. It is also important to get regular exercise and eat healthy food. All these can help you strengthen(增强)your immune system(免疫系统).