The African elephant, which is the largest land animal remaining on earth, is of great importance to African ecosystem. 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 become 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 become 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.
What is the passage mainly about?

A．Disappearance of African elephants.

B．The effect of African elephants’ search for food.

C．Forests and savannas as habitats for African elephants.

D．The eating habit of African elephants.

What does the underlined phrase “setting the terms” most probably mean?

A．Fixing the time.

B．Deciding the conditions.

C．Improving the quality.

D．Worsening the state.

What do we know about the open spaces in the passage?

A．They result from the destruction of rain forests.

B．They provide food mainly for African elephants.

C．They are attractive to plant-eating animals of different kinds.

D．They are home to many endangered animals.

The passage is developed mainly by ______.

A．giving examples

B．pointing out similarities and differences

C．describing the changes in space order

D．showing the effect and then explaining the causes