单选题 Most of the 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 is good for both of them. You may see 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 lairds to do so because they move the cause of discomfort. So although they can live without each other, they do better together.
Sometimes an animal has a plant partner. The relationship develops until the two partners can not live without each other. This is so in the corals (珊瑚) of the sea. In their skins there are tiny plants which act as "dustmen", taking some of the waste products from the corals and giving in return oxygen which the animal needs to breathe. If the plants are killed, or are even prevented from receiving light so that they cannot live normally, the corals will die.

单选题 Some birds like to sit on a sheep because ______.
【正确答案】 A
【答案解析】[考点] 细节题
[解析] 从文章第一段第四句This is not because they want a ride，but because they find easy food in the parasites on sheep.可知它们想吃羊背上的寄生虫。
单选题 The underlined word "they" in the last sentence of the first paragraph refers to ______.
【正确答案】 B
【答案解析】[考点] 细节题
[解析] 从文章第一段得知羊和鸟在一起时各得其所，
单选题 We learn from the text that corals depend on plants for ______.
【正确答案】 D
【答案解析】[考点] 细节题
[解析] 从文章第二段第四句In their skins there are tiny plants which act as“dustmen”，taking some of the waste products from the corals and giving in return oxygen which the animal needs to breathe．得知。
单选题 What does the second paragraph mainly discuss?
【正确答案】 A
【答案解析】[考点] 主旨题
[解析] 从文章最后两句In their skin...needs to breathe．If the plants are killed...the corals will die．说明了动物和植物之间相互依存的关系。