新东方GMAT逻辑推理书上的问题

Although part of the ivory available on world markets was taken from wild elephants that were killed illegally, some ivory is derived from sources that nearly all nations define as legal.such as elephants that have died natural deaths. The world's few remaining wild elephant herds, therefore, are not endangered when those buying ivory at wholesale make a serious effort to limit their purchases to such legal ivory.

The argument above depends on the assumption that

A. wholesale buyers attemping to limit their purchases to legal ivory can reliably distinguish legal from illegal ivory.

B. the demand for products made from legal ivory will continue to grow in the near future.

C. there are currently fewer wholesale sources of legal ivory in the world than there are of illegal ivory.

D. wholesale buyers of ivory products are generally unaware of the reasons for the dwindling world supply of ivory

E. a continued supply of legal ivory is ensured because elephants reproduce in captivity.

答案 A.

但我不懂为什么是A!!! 初学者,望前辈指点!谢谢!