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(伙伴关系) which does good to both of them. You may have noticed some birds sitting on the back 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 do with­out each other. This is so in the corals(珊瑚）of the sea. In their skins they have tiny plants which act as “dustman”, taking some of the waste products from the coral and giving in return oxygen 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.

1.Some birds like to sit on a sheep because _________.

A. they can eat its parasites

B. they consider the sheep as their shelter

C. they enjoy traveling with the sheep

D. they find the position most comfortable

2.The underlined word “they” in the first paragraph refers to _______.

A. birds and parasites                                              B. sheep, birds and parasites

C. parasites and sheep                                            D. birds and sheep

3.It can be learnt from the text that the coral depends on the plants for ________.

A. comfort                                     B. light                         C. oxygen                    D. food     

4.What does the second paragraph mainly discuss?

A. Some plants depend on each other for food.

B. Some animals and plants develop their relationship easily.

C. Some animals and plants depend on each other for existence.

D. Some animals live better together.