题目内容：

Most animals only have animals of a different kind for food. But sometimes two kinds of animals come together in a partnership (伙伴关系) which is good for them. You may have noticed some birds 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 let the birds do so because they can stop the parasites from troubling them. So though they can do with it by themselves, they can do better together with each other.

Sometimes an animal has a plant partner. The relationship develops until the two partners cannot do without each other. This is so in corals of the sea. Inside their bodies they have very small plants, which act as “cleaners”, taking the useless things from the coral and giving oxygen in return. That is what the animal needs to live. If the plants are killed, or are even kept from receiving light so that they cannot live as usual, the corals will die.

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

A. they can eat its parasites    B. they enjoy traveling with the sheep

C. they can’t live without its parasites    D. they want to find the warm place

2.The underlined(画下划线) word “they” in the first paragraph means _________.

A. birds and parasites    B. birds and sheep

C. parasites and sheep    D. birds, parasites and sheep

3.We learn from the passage that corals need plants for _______.

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

4.What is the second paragraph mainly about?

A. Some animals and plants cannot live without each other.

B. Some animals and plants cannot develop their friendship easily.

C. Some plants eat each other.

D. Some animals live better together.