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It absorbs sun’s heat． It is responsible for climate change． It reflects sunlight． It may account for a quarter of global warming over the past century．
sun’s heating on snow surface． soot’s increased absorption of solar energy． carbon-dioxide’s warming effect． snow’s increased reflection of sunlight．
twenty-first twentieth-first twenty-one twentieth-one
The twenty The twentieth Twenty Twentieth
Soot Snow Greenhouse gases Wind
for; twenty for; twentieth as; twenty as; twentieth
Ice sea areas． Areas with black carbon． Areas covered with white snow． Melting glaciers．
X sea ice and insufficient sunlight． light snow cover and sufficient sunlight． heavy snow cover and sufficient sunlight． thick sea ice and insufficient sunlight．
twenty the twentieth twentieth / the twenty
Before the twentieth century. Early in the beginning of the twentieth century. At the dawn of the first day of the twentieth century. After the dawn of the twentieth century.
at the beginning of nineteenth century at the beginning of twentieth century before the nineteenth-century after World War I
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