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the heart can get rid of the poisonous elements.
C.they inhale the smoke less deeply.
D.they, in most cases, pretend to be smokers.
44、(同41题)Tobacco smoke is a mixture____
A.of two poisonous elements.
B.of various poisonous substances.
C.of some irritant gases.
D.of carbon dioxide and water vapour.
45、(同41题)Harmfulness from all forms of smoking to the human body may be arranged in severity sequence as follows:____
A.Cigarette smoking is put first, and next comes cigar or pipe smoking.
B.No 1 killer is cigar smoking, and it’s followed by cigarette or pipe smoking.
C.Cigar or pipe smoking has nearly no effect on the human body in contrast with igarette smoking.
D.All forms of smoking are similar in effect harmful to the human body.
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 Impac