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
We learn from the text that corals depend on plants for ______.
A．comfort
B．light
C．food
D．oxygen
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