Years ago, many zoos kept all kinds of animals in small cages. Small cages made it easy for people to see the animals, but a small cage is not a good place for an animal to live in. Today zoos keep animals in different kinds of cages. The cages are very big and open. They usually have plants and little lake. The cages look like the animals' habits. Zoos help to protect all kinds of animals. They protect animals in the zoo and they protect animals in the wild. How do they do this? Zoos teach people how animals live in the wild. Zoos want people to help protect the animals' wild habitats. Many plants and animals are going extinct (灭绝). Mammoths, which related with the Asian elephants, are now extinct. There are no mammoths in the world today. Scientists say that seventy-four different kinds of living things go extinct every day! Zoos are working together to stop animals from going extinct.

An animal or a plant that is going extinct _____.

A.has more and more living members

B.comes into this world soon

C.becomes very dangerous

D.has fewer and fewer living members