Most animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however, two kinds of animals come together in a partnership  which does good to both of them. You may have noticed some birds sitting on the back of sheep. This is not because they want a ride, but because they find easy food in the parasites on sheep. The sheep allow the birds to do so because they remove the cause of discomfort. So although they can manage without each other, _ do better together. Sometimes an animal has a plant partner. The relationship develops until the two partners cannot do without each other. This is so in the corals 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. It can be learnt from the text that the coral depends on the plants for   _  .
comfort light oxygen food