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  3. Many people believe the glare from snow causes snow blindness. Yet, with dark glasses or not, they find themselves suffering fro

Many people believe the glare from snow causes snow blindness. Yet, with dark glasses or not, they find themselves suffering fro

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问题     Many people believe the glare from snow causes snow blindness. Yet, with dark glasses or not, they find themselves suffering from headaches and watering eyes, and even snow blindness, when exposed to several hours of "snow light".
    The United States Army has now determined that the glare from snow does not cause snow blindness in troops in a snow-covered country. Rather, a man’s eyes frequently find nothing to focus on in a broad expanse of a snow-covered area. So his gaze continually shifts and jumps back and forth over the entire landscape in search of something to look at. Finding nothing, hour after hour, the eyes never stop searching and the eyeballs become sore and the eye muscle aches. Nature balances this annoyance by producing more and more liquid which covers the eyeballs. The liquid covers the eyeballs in increasing quantity until vision blurs. And the result is total, even though temporary, snow blindness.
    Experiments led the Army to a simple method of overcoming this problem. Scouts ahead of a main body of troops are trained to shake snow from evergreen bushes, creating a dotted line as they cross completely snow-covered landscape. Even the scouts themselves throw lightweight, dark-colored objects ahead on which they too can focus. The men following can then see something. Their gaze is arrested. Their eyes focus on a bush and having found something to see, stop searching through the snow-blanketed landscape. By focusing their attention on one object at a time, the man can cross the snow without becoming hopelessly snow blind or lost. In this way the problem of crossing a solid white area is overcomed.
To prevent headache, watering eyes and blindness caused by the glare from snow, dark glasses are______.
选项 A、indispensable
B、useful
C、ineffective
D、available
答案C
解析 细节题。短文第一段第二句指出Yet，with dark glasses or not they findthemselves suffering from headaches and watering eyes，and even snow blindness即不管带不带墨镜，人还是会感到头痛，眼睛变湿，甚至产生雪盲。由此可知墨镜在这种情况下是没有效果的，故选项C正确。
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