单选题 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(糖浆) 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 Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones.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.
单选题 Which of the following is true according to the passage?
【正确答案】 A
【答案解析】[解析] 根据文中第一段“Rod-shaped bacteria are usually two to four microns long, while rounded ones are generally one micron in diameter”，可明显得出rod-shaped bacteria比rounded bacteria要大，故A正确。
单选题 According to the passage, one who examines bacteria with an ordinary microscope, using a magnification of 100 times will see ______.
【正确答案】 A
【答案解析】[解析] 根据文中第二段前两句“Using a magnification of 100 times, one finds that bacteria are barely visible as tiny rods or dots”，故选A。
单选题 The environment around bacteria is always changing because ______.
【正确答案】 A
【答案解析】[解析] 根据文章第三段“Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones”，故选A。
单选题 The author compares water to molasses in order to show ______.
【正确答案】 A
【答案解析】[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。
单选题 The relationship between a bacterium and its flagella is most similar to which of the following?
【正确答案】 D
【答案解析】[解析] 根据文章第二段最后几句“...some bacteria have attached to them wavy-looking "hairs" called flagella... The flagella rotate, pushing the bacteria through the water.”可知D更确切。