Zoos are under a lot of pressure these days to justify their existence. In the past, collections of animals were assembled largely to entertain the public or to satisfy a private desire. There was often little understanding of the animal's biological or behavioral needs, and even less perception of a responsibility to justify the collection in terms of science, conservation or popular education. For responsible zoos, this has changed dramatically.
By acting as a reservoir for species, zoos can act as a safety device in case the species become extinct in nature. Zoos can also provide the animals necessary to reintroduce species extinct in the wild once the threats to their survival have been controlled. A second important function of any good zoo must be the education of the visitor. Regardless of the scientific expertise (专长) of the individual, he or she can expect to learn something every time he walks through the zoo gate.
For many people, especially those living in towns, this is the closest they will come to see wild animals. Even with the high quality of the current wildlife films, there is no replacement for seeing the real thing. One must appreciate that for many people an animal simply isn't real if they only see it on a television screen. Zoos provide an opportunity to make the emotional and rational commitment essential to the future of conservation.
Finally, it should be appreciated that zoos provide a unique opportunity for research, the results of which may be crucial to the survival of species.
Apart from the debate over whether zoos perform. a valuable role in society, they are often attacked on animal welfare grounds -- that they are little more than prisons, exploiting their inhabitants for commercial purposes. Unfortunately there is a tendency to tar (玷污) all zoos with the same brush, regardless of fundamental differences in their philosophy, the quality of care given to their animals, and their contribution to science and conservation.
Nobody is arguing that zoos are perfect or that they are a substitute for the wild. But it is important to place them -- the responsible, good zoos -- in the overall context of conservation, and to recognize that they and the staff they support have a valuable role to play in the future.
What is the author's argument in the passage？

An important new role has evolved for good zoos: they are not prisons for animals but reservoirs of species.
B. Zoos should learn to justify their existence no matter how great the pressure is.
Criticism on zoos ought to be more specific because zoos are not graded on the same level.
D. The function of education of zoos becomes more important and practical.