单项选择题

The underlined word "they" in the last sentence of the first paragraph refers to ____________.

A．birds and parasites
B．birds and sheep
C．parasites and sheep
D．sheep, birds and parasites
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单项选择题
Some birds like to sit on a sheep because ____________.
A．they can eat its parasites
B．they enjoy travelling with the sheep
C．they depend on the sheep for existence
D．they find the position most comfortable
单项选择题
What does the second paragraph mainly discuss
A．Some animals and plants depend on each other for existence.
B．Some animals and plants develop their relationship easily.
C．Some plants depend on each other for food.
D．Some animals live better together.
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