46、Vision。 I____。Vision is physiological power of sight. Vision is the faculty by which, th rough the material organ, the eye, the visible external world is perceived. Many simple organisms have light receptors and can thus react to motion and shadows, but true vision involves the formation of images in the brain. Eyes of differen t organisms provide images of varying clarity: This article deals with vision in humans and other animals with eyes of comparable complexity.

47、Basic P____。On ordinary optical principles, a point above the direct line of vision co mes to a focus at a point of the retina below its center, and vice versa. If a r etina were observed by another person, the observer would see that the image of an object is formed on the retina, and that this image is inverted. Any increase in the magnitude of the retinal image is generally associated with approach of the object; if this same result is brought about by means of lenses, however-eve n where the real distance is increased-the object still seems to approach. This seeming approach is the result of an unconscious process of reasoning. The mind interprets any given object as being of a known or ascertained size.

48、Sight D____。The most common disturbance to vision is caused by crystals or other small opaque bodies in the humors of the eye, which are usually only a passing inconv enience. Much more serious are opacities called cataracts, which develop in the lens as a result of mechanical injury, advancing age, or dietary deficiencies. O pacity of the cornea also causes obscured vision, but this condition may be repa ired through transplantation of a section of clear cornea from another person.

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.

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 

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