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  3. Bacteria are microscopic organisms which live on the surface of objects. They are one of the most ancient living things, which e

Bacteria are microscopic organisms which live on the surface of objects. They are one of the most ancient living things, which e

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问题     Bacteria are microscopic organisms which live on the surface of objects. They are one of the most ancient living things, which exist on this planet for nearly 4, 000 million years. Do you know the size of bacteria? It can only be measured in microns. Maybe you have not a standard in your mind how long a micron is. One micron is a thousandth of a millimeter, which equals about the diameter of a pinhead. Therefore, even if we enlarged the rounded bacterium a thousand times, it would only be the size of a pinhead. We barely see bacteria by a magnification an ordinary microscope of 100 times, even if we try, we cannot make out anything of their structure. There are normally millions of them together, for they can multiply really fast.
    Scientists have found that some bacteria have attached to wavy-looking "hairs" called flagella. The flagella rotate, pushing the bacteria through the water. Others can glide along over surfaces by some little-understood mechanism. Bacteria are so small that they are influenced by the movements of the chemical molecules around them. They are active all the time. Even the bacteria without flagella often bound about in the water. They are pushed here and there, colliding with the watery molecules.
    Bacteria cannot be detected because they don’t produce bad odor or change the color or texture of the food. Therefore, when people eat the food with many bacteria, they are likely to get hepatitis A, acute gastroenteritis and a host of other illnesses. Many households have refrigerators to prevent from bacteria. Of course, freezing food slows or stops-the growth of bacteria, however, when food is thawed, the bacteria will become reactivated. Bacteria can not be totally destroyed before the food is thoroughly cooked. We need to know that not all bacteria are in connection with illness. Just some bacteria can cause disease. They are called pathogenic bacteria. Fortunately our immune system can protect us from them.
    Bacteria are prokaryotes (single cells that do not contain a nucleus). It may seem weird to classify organisms according to such details, but with or without a nucleus is not trivial at all. The division between prokaryotes and eukaryotes (all organisms with a nucleus inside their cells) is of extreme importance in biology, and is the result of a major evolutionary breakthrough. Visit our program tomorrow if you want to know more, thank you.
Which of the following is true according to the first paragraph?
选项 A、Bacteria are as big as a pinhead.
B、Bacteria can not be seen by an ordinary microscope.
C、Bacteria are hard to see because they often live together.
D、Bacteria are very small so they can only be measured in microns.
答案D
解析 根据第一段中第四句“It can only be measured in microns．”可见，细菌只能以微米来计算，故D项正确。根据该段第六句“One micro is a thousandth of a millimeter，which equals about the diameter of a pinhead．”可见，一微米等于千分之一毫米，一毫米大约等于针头的直径，即细菌的大小相当于针头直径的千分之一，故A项错误。根据该段第八句“We barely see bacteria by…”可见，用普
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