Most animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however, two kinds of animals come together in a partnership which does good to both of 题目和参考答案——青夏教育精英家教网—

题目内容

Most animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however, two kinds of animals come together in a partnership which does good to both of them. You may have noticed some birds sitting on the backs of sheep.This is not because they want a ride, but because they find easy food in the parasites(寄生虫) on sheep. The sheep allow the birds to do so because they remove the cause of discomfort. So although they can manage without each other, they do better together.

Sometimes an animal has a plant partner. The relationship develops until the two partners cannot manage without each other. This is so in the corals of the sea. In their skins they have tiny plants which act as "dustmen", taking some of the waste products from the coral and giving in return oxygen(氧气) which the animal needs to breathe.

If the plants are killed, or are even prevented from light so that they cannot live normally, the corals will die.

13. Some birds like to sit in a sheep because ________.

A. they can eat its parasites

B. they depend on the sheep for living

C. they enjoy travelling with the sheep

D. they find the position most comfortable

14. The word "they" in the last sentence of the first paragraph refers to _______ .

A. birds and parasites

B. birds and sheep

C. sheep and parasites

D. sheep, birds and parasites

15. From the passage we know that the coral is a kind of ________.