B. Read the passage carefully, then choose the correct answer.

In today’s world, thousands of plants and animals are in danger of extinction, and the number of species at risk increases every year. Not all plants or animals existing in small numbers are endangered as their population may not be decreasing or threatened. An endangered species is one which is expected to die out within twenty years unless special measures are taken to protect it. Endangered animals include blue whales, giant pandas, orang-outans, rhinoceroses, snow leopards, tigers, and some species of crocodile.

Endangered plants include some species of cactus. Why are the above species of animal endangered? There are now legal limits on hunting, but not all species protected are by law. Even when animals are legally protected, illegal hunting for fur, tusks and horn continues. Those animals that are not killed are captured and sent to zoos or sold as pets.

For example, the little blue macaw, a parrot native to Brazil, has been captured in such numbers for private bird collectors that it is almost extinct in the wild. In addition to this, many species‘ habitats are eliminated when people clear land in order to build on it, or when they introduce domestic and non-native animals which overgraze the vegetation and even prey on birds and mammals which are already at risk of extinction.

Why is it important to save endangered species? Firstly, nature is delicately balanced, and the extinction of one species may have a serious effect on others. Secondly, it is the fault of mankind that so many species are endangered, so it is our responsibility to protect those species while we still can. We must stop the hunting of wild animals and the destruction of the forests before it is too late. It is worth making an effort - by saving endangered species we could be saving ourselves.

According to the author, all the species mentioned are in danger because ______