阅读下面短文,用英语回答文后所给的5个问题。
Bacteria(细菌) are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial size is measured in micron. One micron is a thousandth of a millimeter; a pinhead is about a millimeter across. Rod-shaped bacteria are usually from two to four microns long, while rounded ones are generally one micron across. Thus, if you magnified a rounded bacterium a thousand times, it would be just the size of a pinhead, while a grown-up human enlarged by the same amount would be over a mile tall.
Even with an ordinary microscope(显微镜), you must look closely to see bacteria. Using a magnification of 100 times, one can hardly find bacteria. Nor can one make out anything of their structure(结构), of course. Only by using special colors, can one see that some bacteria have wavy-looking “hairs” called flagella. Others have only one flagellum. The flagella move round a central point, pushing the bacteria through the water. Many bacteria lack flagella and cannot move about by their own power, while others can move along over surface by some little-understood “machinery”.
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(糖浆) is to us. Bacteria are so small that they are affected by the movements of the chemical molecules(分子) around them. Bacteria under microscopes, even those with no flagella, often jump up and down in the water. This is because they knock with the water molecules and are pushed this way and that.
47. What does the underlined word “magnified” mean?
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48. What is the smallest according to the passage?
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49. What is the relationship between a bacterium and its flagella is most nearly like?
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50. Why does the writer compare water to molasses in the third paragraph?
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