【单选题】

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 InternationalDark SkyAssociation, a nonprofit that aims to control light pollution.

Last year, in Sydney, an estimated 2.2 millionAustralians 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 see. In dark rural areas, about 2,000 stars are typically visible at night, compared with "maybe five" in a bright city square—and about 5,000 in centuries past.
People who are working while others are star-gazing may face the greatest risks. Nighttime exposure to white light can cause the growth of tumors (肿瘤), experiments show. Two decades of research indicate that women who work at night shifts have unusually high rates of breast cancer.
On March 29 this year people in many cities around the world ______.A．turned out some lights for an hour
B．organize some events to have some fun
C．held an demonstration called "Earth Hour"
D．joined together to compete withAustralians