Bacteria(细菌) are extremely small living things. While we measure our own sizes in inches

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.
36. The underlined word magnified means     .
A. enlarged                                                      B. widened                  
C. killed                                                          D. caught
37. We know from the passage that      is the smallest.
A. a pinhead                                            B. a rounded bacterium 
C. a microscope                                        D. a rod-shaped bacterium
38. The relationship between a bacterium and its flagella is most nearly like which of the following?
A. A rider jumping on a horse back 
B. A ball being hit by a bet
C. A boat powered by a motor    
D. A door closed by wind
39. Why does the writer pares water to molasses in the third paragraph?
A. To tell us how difficult it is for bacteria to move through water.
B. To suggest that bacteria are fond of different liquids.
C. To show different chemicals are of different structures.
D. To show that bacteria are the best swimmers.
40. Which of the following is the main topic of the passage?
A. The characteristic (特点) of bacteria.      
B. How bacteria reproduce.
C. The various parts of a bacterium’s body    
D. How bacteria cause diseases.            A