Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

There are three valid arguments to support the preservation of endangered species. An aesthetic justification contends that biodiversity contributes to the quality of life because many of the endangered plants and animals are particularly appreciated for their unique physical beauty. The aesthetic role of nature in all its diverse forms is reflected in the art and literature of every culture, attaining symbolic status in the spiritual life of many groups. According to the proponents of the aesthetic argument, people need nature in all its diverse and beautiful forms as part of the experience of the world.

Another argument that has been put forward, especially by groups in the medical and pharmacological fields, is that of ecological self–interest. By preserving all species, we retain a balance of nature that is ultimately beneficial to humankind. Recent research on global ecosystems has been cited as evidence that every species contributes important or even essential functions that may be necessary to the survival of our own species. Some advocates of the ecological argument contend that important chemical compounds derived from rare plants may contain the key to a cure for one of the diseases currently threatening human beings. If we do not protect other species, then they cannot protect us.

Apart from human advantage in both the aesthetic and ecological arguments, the proponents of a moral justification contend that all species have the right to exist, a viewpoint stated in the United Nations World Charter for Nature, created in 1982. Furthermore, if humankind views itself as the stewards of all the creatures on Earth, then it is incumbent upon human beings to protect them, and to ensure the continued existence of all species. Moral justification has been extended by a movement called "deep ecology", the members of which rank the biosphere higher than people because the continuation of life depends on this larger perspective. To carry their argument to its logical conclusion, all choices must be made for the biosphere, not for people.

(Source: Adapted from Reading Practice for the TOEFL) 

The word "them" in the third paragraph refers to ________.