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 from 35 to 42.

  The idea of preserving biological diversity gives most people a warm feeling inside. But what, exactly, is diversity? And which kind is most worth preserving? It may be anathema to save-the-lot environmentalists who hate setting such priorities, but academics are starting to cook up answers.

  Andrew Solow, a mathematician at the Woods Hole Oceanographic Institution, and his colleagues argue that in the eyes of conservation, all species should not be equal. Even more controversially, they suggest that preserving the rarest is not always the best approach. Their measure of diversity is the amount of evolutionary distance between species. They reckon that if choices must be made, then the number of times that cousins are removed from one another should be one of the criteria.

  This makes sense from both a practical and an aesthetic point of view. Close relatives have many genes in common. If those genes might be medically or agriculturally valuable, saving one is nearly as good as saving both. And different forms are more interesting to admire and study than lots of things that look the same. Dr Solow’s group illustrates its thesis with an example. Six species of crane are at some risk of extinction. Breeding in captivity might save them. But suppose there were only enough money to protect three. Which ones should be picked?

  The genetic distances between 14 species of cranes, including the six at risk, have already been established using a technique known as DNA hybridisation. The group estimated how likely it was that each of these 14 species would become extinct in the next 50 years. Unendangered species were assigned a 10% chance of meeting the Darwinian reaper-man; the most vulnerable, a 90% chance. Captive breeding was assumed to reduce an otherwise endangered species’ risk to the 10% level of the safest. Dr Solow’s computer permed all possible combinations of three from six and came to the conclusion that protecting the Siberian, white-naped and black-necked cranes gave the smallest likely loss of biological diversity over the next five decades. The other three had close relatives in little need of protection. Even if they became extinct, most of their genes would be saved.

  Building on the work of this group, Martin Weitzman, of Harvard University, argues that conservation policy needs to take account not only of some firm measure of the genetic relationships of species to each other and their likelihood of survival, but also the costs of preserving them. Where species are equally important in genetic terms, and - an important and improbable precondition - where the protection of one species can be assured at the expense of another, he argues for making safe species safer, rather than endangered species less endangered.

  In practice, it is difficult to choose between species. Most of those at risk - especially plants, the group most likely to yield useful medicines - are under threat because their habitats are in trouble, not because they are being shot, or plucked, to extinction. Nor can conservationists choose among the millions of species that theory predicts must exist, but that have not yet been classified by the biologists assigned to that tedious task.

  This is not necessarily cause for despair. At the moment, the usual way to save the genes in these creatures is to find the bits of the world with the largest number of species and try to protect them from the bulldozers. What economists require from biologists are more sophisticated ways to estimate the diversity of groups of organisms that happen to live together, as well as those which are related to each other. With clearer goals established, economic theory can then tell environmentalists where to go.

[from The Economist]

Endangered species of cranes can be saved by _____________

A. stopping hunters from killing them 

B. protecting their habitats 

C. encouraging them to mate with their cousins 

D. keeping them in zoos or wildlife parks