Most animals have little connection with animals of a different kind, unless they hunt the
Sometimes an animal has a plant partner. The relationship develops until the two partners cannot manage without each other. This is so in the corals (珊瑚) of the sea. In their skins they have tiny plants which act as "dustman", taking some of the waste products from the coral and giving in return oxygen which the animal needs to breathe. If the plants are killed, or are even prevented from light so that they cannot live normally, the corals will die.
Some birds like to sit on a sheep because ______.
A.they can eat its parasites
B.they depend on the sheep for existence
C.they enjoy travelling with the sheep
D.they find the position most comfortable