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 batter together.

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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

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