49、W____of Vision。Day blindness, or hemeralopia, is caused by incipient clouding or opacity of one or more of the ocular tissues. Night blindness, or nyctalopia, results fr om a deficiency of visual purple in the retina caused by lack of vitamin A. Colo r blindness is attributable to a congenital defect in the retina or other nerve portions of the optic tract. Amblyopia is weakness of vision without apparent st ructural damage of the eye and may be caused by poisoning by drugs, alcohol, or tobacco or may be associated with hysteria or uremia.

50、B ____。Pressure on the optic nerve may cause blindness in the right or left halve s of both eyes or in the inner or outer halves of both eyes. Detachment of the r etina from the interior of the eyeball causes blindness by allowing the retina t o drift to the bottom of the eye out of range of the image formed by the lens; p ermanent correction usually requires surgery. Introduction Principles Defects Weakness Blindness

51、Environmental Impacts of Energy Production All primary energy sources that are ***nomic will be required if the world is to avoid energy starvation. The world’s needs for e(51)____ energy have been met successively by wood, coal and oil. As these sources decline in importance o(52)____ approach depletion, natural gas, nuclear power, solar and other ’alternative’ energy sources are expected to m(53)____ an increasing fraction of total demand. The production and use of energy can h(54 )____ a significant effect on the environment. The environmental consequences of the use of some energy sources c ould conceivably b***me so great t(55)____they could limit growth in energy con sumption. On the other hand, meeting energy needs for ***nomic development and s(56)____well-being may require that some environmental impcts be accepted. Environmental impact assessments are being used increasingly I(57)____ dec ision-making by national authorities r(58)____ for selecting appropriate energy options or energy mixes. The impacts of energy production may be of short durati on, long lasting, or irreversible. They may be of l(59)____or global concern, and may involve trans-boundary issues of international significance; they may af fect different communities, p(60)____ groups and countries in different ways. Th ey may be detrimental or beneficial. The nuclear industry has taken a leading ro le in addressing systematically both short- and long-term issues.
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答案：

1　 D  2　 B  3　 A  4　 B  5　 B  6　 D  7　 B  8　 D  9　 C  10　 C  11　 A  12　 C  13　 A  14　 D 
15　 C  16　 C  17　 C  18　 A  19　 B  20　 B  21　 C  22　 A  23　 C  24　 B  25　 A  26　 D  27　 C  28　 D  29　 B  30　 A  31　 C  32　 C  33　 B  34　 B  35　 C  36　 B  37　 C  38　 A  39　 D  40　 B  41　 B  42　 A  43　 C  44　 B  45　 A  46　 Introuction  47　 Principles  48　 Defects  49　 Weakness 
50　 Blindness  51　 energy  52　 or  53　 meet  54　 have  55　 that  56　 social  57　 in  58　 responsible 
59　 local  60　 population