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
We learn from the text that corals depend on plants for
A．comfort
B．light
C．food
D．oxygen
单项选择题
Some birds like to sit on a sheep because______.
A．they can eat its parasites
B．they enjoy traveling with the sheep
C．they depend on the sheep for existence
D．they find the position most comfortable
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