[单项选择]The night is not what it was. Once, the earth was cast half in shadow. Then came fire, candle, and light bulb, gradually drawing back the curtain of darkness. But a brighter world has its drawbacks. An estimated 30 percent of outdoor lighting—plus even some indoor lighting—is wasted. Inefficient lighting costs U.S. about $10.4 billion a year, according to bob Gent of the International Dark Sky Association, a nonprofit that aims to control light pollution.
Last year, in Sydney, an estimated 2.2 million Australians switched off their lights during "Earth Hour", briefly reducing that city’s energy use by more than 10 percent. Motivated by such trends, more than two dozen cities worldwide went dim on March 29 this year in an hour-long demonstration.
A number of groups are trying to measure light pollution and assess its effect on the environment in the hope that people will reduce their own contribution to the problem. Scientists are trying to report how many stars we can
A. lose the chance to gaze stars
B. have a higher risk of health problem
C. have less exposure to white light
D. are able to blame for light pollution

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