题目内容：
Questions are based on the following passage．There can be few more depressing stories in the entire history of man's exploitationof nature than the destruction of the unfortunate great whales. The whales have notonly suffered untold cruelty but now face total extermination ( 灭绝 ) . Already entirepopulations have been wiped out, and the only reason why no species has yet beenfinished off is due to the vastness and inaccessibility (难到达) of the oceans; a pocketor two somewhere has always managed to escape.
The basic rule of extinction is very simple: it occurs when a species mortality iscontinually greater than its recruitment. There are though, some very special additionalfactors in the case of whales.
Man does not actually have to kill the last whales of a species with his own hands,as it were, to cause its disappearance. Biological extinction will quickly follow the end ofcommercial whaling, should that end be due to a shortage of raw material, i.e. of whales.
Whalers have long sought to defend their wretched trade by insisting that whales areautomatically protected: as soon as they become rare, and therefore uneconomic to pursue,man will have no choice but to stop the hunting. That is a very nice theory, but it is thetheory of an accountant and not of a biologist; only an accountant could apply commercialeconomics to complex biological systems. The reasons for its absurdity are many andvaried. In the case of whaling it can be summed up in the following way.
When the stock has been reduced below a critical level, a natural, possiblyunstoppable downward spiral begins because of three main factors. First, the animalslucky enough to survive the slaughter will be too scattered to locate one another owningto the vastness of the oceans. Secondly, whales being sociable animals probably needthe stimulus of sizeable gatherings to induce reproductive behavior. It is quite likely thattwo individuals meeting through chance will not be compatible. (They can hardly beexpected to be aware of their own rarity or to realize any need for adjusting their naturalinclinations.) This is especially so with polygamous ( 一夫多妻的 ) species like theSperm Whale. Thirdly, and perhaps most important in the long term, even allowing thatthe whales might still be able to band together in socially acceptable groups (thanks totheir undeniably excellent communicative systems), there is a real danger that the whales'gene pools would by then have sunk so low as to be biologically unviable. That is to say,the characteristics possessed by the original population would be whittled ( 削弱 ) downto those characters possessed by only the few remaining individuals. The result of sucha biological disaster is inbreeding ( 近亲繁殖 ) , less ability to adapt to new conditions,and less individual variety. Three words can sum it up: protracted ( 长时间的 ) biologicalextinction.
According to the passage, the extinction of whales is caused by ___________. A．man's continuous evolution process
B．people's excessive hunting and killing
C．the changing living conditions in nature
D．the demand from a booming economy
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