A
Everything living on earth--plants and animals need other living things. Nothing lives alone. Most animals must live in a group, and even a tree or a plant grows close together with others of the same kind. Sometimes one living thing hunts another; one eats and the other is eaten. Each kind of life eats another kind of life in order to live, and together they form a food chain. Some food chains are simple; others are complicated. But all have two things in common--all food chains begin with the sun, and all food chain become broken up if one of the links disappears. All life depends on energy from sunlight. Only plants can use this energy directly. Their leaves are little factories that use sunlight to make food from water and things in the soil and air.
Plants in turn feed all other living things. Animals can only use the sun’s energy after it has been changed into food by plants. Some animals feed directly on plants; others eat smaller animals. Me
A. from water
B. from the air in the sky
C. from the soil
D. from water, things in the soil and air and sunlight
Bacteria (细菌) 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 from 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. Others have only one flagellum. The flagella rotate, pushing the bacteria through the water. Many
A. tiny dots
B. small "hairs"
C. large rods
D. detailed structures
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