文档介绍：The African elephant, the largest land animal remaining on earth, is of great importance to African ecosystem(生态系统). Unlike other animals, the African elephant is to a great extent the builder of its environment. As a big plant-eater, it largely shapes the forest-and-savanna(大草原)surroundings in which it lives, therefore setting the terms of existence for millions of other animals that live in its habitat.
It is the elephant’s great desire for food that makes it a disturber of the environment and an important builder of its habitat. In its continuous search for the 300 pounds of plants it must have every day, it kills small trees and under-bushes, and pulls branches off big trees. This results in numerous open spaces in both deep tropical forests and in the woodlands that cover part of the African savannas. In these open spaces are numerous plants in various stages of growth that attract a variety of other plant-eaters.
Take the rain forests for example. In their natural state, the spreading branches overhead shut out sunlight and prevent the growth of plants on the forest floor. By pulling down trees and eating plants, elephants make open spaces, allowing new plants to grow on the forest floor. In such situations, the forests e suitable for large hoofed plant-eaters to move around and for small plant-eaters to get their food as well.
What worries scientists now is that the African elephant has e an endangered species. If the elephant disappears, scientists say, many other animals will also disappear from vast areas of forest and savanna, greatly changing and worsening the whole ecosystem.
21. What does the underlined phrase “setting the terms” most probably mean?
A. Improving the quality. B. Worsening the state. C. Fixing the time. D. Deciding the conditions.
22. The passage is developed mainly by_________.
A. showing the effect and then explaining the causes B. giving examples
C. pointing out similarities and differences D. describing the changes in sp