2.        Among the more somber gifts brought by the Enlightenment was the realization that humansmight one day become extinct. The astronomical revolution of the 17th century had shown that thesolar system both operated according to the highest principles of reason and contained comets whichmight __21__ hit the Earth. The geological record, as interpreted by the Comte de Buffon, showedmassive extinctions in which species vanished forever. That set the scene for Charles Darwin torecognize such extinctions as the motor of evolution, and thus as both the force which had __22__ humans and, by implication, their possible destiny. The nascent science of thermodynamics added acosmic dimension to the certainty of an ending； Sun, Earth and the whole__23__ would eventually run down into a lifeless “heat death”.      The 20th century added the idea that extinction might not come about naturally, but throughartifice. The spur for this was the discovery, and later exploitation, of the power locked up in atomicnuclei.__24__ some of its discoverers as a way of indefinitely deferring heat death, nuclear energywas soon developed into a far more proximate danger. And the tangible threat of imminentcatastrophe which it posed__25__ on other technologies. None was more tainted than the computer.It may have been guilt by association: the computer played a vital role in the development of thenuclear arsenal.
【題組】21.
(A) permanently
(B) conceivably
(C) rigorously
(D) intrinsically