根据上下文语境和首字母提示, 在划线处写出—个合适的词。

Are we endangered?

Steve Jones is an environmental expert who tries to keep animals and plants from becoming endangered. If we know more about what causes e【小题1】, we may be able to take m【小题2】 before it is too late. A species can become e【小题3】 for different reasons. Animals and plants must have a habitat or home, which is c【小题4】 and clean, and where there is enough food and other resources. They are all used to their environment; that is, they have learnt how to live s【小题5】 in their habitat.