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  3. Bacteria are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial size is measured

Bacteria are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial size is measured

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问题     Bacteria are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial size is measured in microns. One micron is a thousandth of a millimeter; a pinhead is about a millimeter across. Rod-shaped bacteria are usually two to four microns long, while rounded ones are generally one micron in diameter. Thus, if you enlarged a rounded bacterium a thousand times, it would be just about the size of a pinhead. An adult human magnified by the same amount would be over a mile (1.6 kilometers) tall.
    Even with an ordinary microscope, you must look closely to see bacteria. Using a magnification of 100 times, one finds that bacteria are barely visible as tiny rods or dots. One cannot make out anything of their structure. Using special stains, one can see that some bacteria have attached to them wavy-looking "hairs" called flagella (鞭毛). Others have only one flagellum. The flagella rotate, pushing the bacteria through the water. Many bacteria lack flagella and cannot move about by their own power, while others can glide along over surfaces by some little-understood mechanism.
    From the bacterial point of view, the world is a very different place from what it is to humans. To a bacterium, water is as thick as molasses is to us. Bacteria are so small that they are influenced by the movements of the chemical molecules (分子) around them. Bacteria under the microscope, even those with no flagella, often bounce about in the water. This is because they collide with the water molecules and are pushed this way and that.
    Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones. Even bacteria without flagella are thus constantly exposed to a changing environment.
The author compares water to molasses in order to show______.
选项 A、how difficult it is for bacteria to move through the water
B、how different the bacterial world is from the human world
C、how the water molecules work
D、what different bacteria one liquid contains from another
答案A
解析 由文章第三段前三句可知作者将水比作糖浆是为了说明“水对于细菌来说就相当于糖浆对于人类一样稠，所以细菌在水中运动起来就很困难”，故选A。
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