题目内容：
回答题。Everything living on earth--each plant and animal--needs other living things．Nothing lives a-lone．Most animals must live in a group, and even a tree or a plant grows close together with oth-ers of the same kind．Sometimes one living thing hunts another, one eats and the other is eaten．
Each kind of life eats another kind of life in order to live, and together they form a food chain．
Some food chains are simple, others are complicated．But all have two things in common--all foodchains begin with the sun, and all food chains become broken up if one of the links disappears．
All life depends on energy from sunlight．Only plants can use this energy directly．Their leav-es are little factories that use sunlight to make food from water and things in the soil and air．
Plants in turn feed all other living things．Animals can only use the sun' s energy after it hasbeen changed into food by plants．Some animals feed directly on plants, others eat smaller ani-mals．Meat-eating animals are only eating plants indirectly．
What about human beings? We are members of many food chains．We eat wheat, rice, vege-tables, fruits and so on．We also eat meat and drink milk．This means the sun' s energy passesthrough plant to animal before it reaches us．
Nature is a greater thing．Any food chain always produces enough for each of its members if itis left alone．When there isn' t enough food for any link in the chain, some of its members die off．
So the balance is always kept．
But men in their greed and ignorance often break up the food chain and do great harm not onlyto one plant or animal, but to all the links in the chain．People make seas and rivers dirty．Theydestroy whole forests and kill many kinds of wild animals and birds．When a fiver becomes dirty,the fish cannot be eaten．Men eat the fish and get strange diseases．In some places men have nofish to eat any more, because the fish have died off．Each form of life is linked to all others．
Breaking the links puts all life in danger．
How does everything living on earth live? A．Each plant can live alone．
B．Each animal can live alone．
C．Everything living on earth can not live without needing other living things．
D．If living things want to live they must kill each other．
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