Most of the animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however,

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问题     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 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 me parasites (寄生虫) on sheep. The sheep allow the birds to do so because they can remove the cause of discomfort. So although mey can live without each other, they do better together.
    Sometimes an animal has a plant partner. The relationship develops until the two partners cannot live without each other. This is so in me corals(珊瑚)of the sea. In their skins mere are tiny plants which act as "dustmen" , taking some of the waste products from me corals and giving in return oxygen(氧气) 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.
The underlined word "they" in the last sentence of the first paragraph refers to______.

选项 A、birds and parasites
B、birds and sheep
C、parasites and sheep
D、sheep, birds and parasites

答案B

解析 从文章第一段“The sheep allow the birds to do so because they can remove the cause of discomfort. So although they can live without each other they do better together.”可知,这里的“they”指的是鸟和羊。故正确的答案为B。
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