阅读下面这篇短文，短文后有2项测试任务；(1)第23～26题要求．从所给的6个选项中为第2～5段每段选择1个正确的小标题；(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Soot and Snow: a Hot Combination
1 New research from NASA scientists suggests emissions of black soot alter the way sunlight reflects off snow. According to a computer simulation, black soot may be responsible for 25 percent of observed global warming over the past century.
2 Soot in the higher latitudes of the Earth, where ice is more common, absorbs more of the sun’s energy and warmth than an icy, white background. Dark-colored black carbon, or soot, absorbs sunlight, while lighter colored ice reflects sunlight.
3 Soot in areas with snow and ice may play an important role in climate change. Also, if snow and ice covered areas begin melting, the warming effect increases, as the soot becomes more concentrated on the snow suce. "This provides a positive feedback, as glaciers and ice sheets melt, they tend to get even dirtier," said Dr. James Hansen, a researcher at NASA’s Goddard Institute for Space Studies, New York.
4 Hansen found soot’s effect on snow albedo (solar energy reflected back to space), which may be contributing m trends toward early springs in the Northern Hemisphere, such as thinning Arctic sea ice, melting glaciers and permafrost. Soot also is believed to play a role in changes in the atmosphere above the oceans and land.
5 "Black carbon reduces the amount of energy reflected by snow back into space, thus heating the snow suce more than if there were no black carbon," Hansen said. Soot’s increased absorption of solar energy is especially effective in warming the world’s climate. "This forcing is unusually effective, causing twice as much global warming as a carbon-dioxide forcing of the same magnitude," Hansen noted.
6 Hansen cautioned, although the role of soot in altering global climate is substantial, it does not alter the fact that greenhouse gases are the primary cause of climate warming during the past century, Such gases are expected to be the largest climate forcing for the rest of this century.
7 The researchers found that observed warming in the Northern Hemisphere was large in the winter and spring at middle and high latitudes. These observations were consistent with the researchers’ climate model simulations, which showed some of the largest warming effects occurred when there were heavy snow cover and sufficient sunlight. Hansen cautioned that greenhouse gases ______.