D absorb more of sun’s energy and warmth than white background
E still surpass soot in warming the world’s climate during the last century
F can be seen mostly in the Northern Hemisphere
5. In the twentieth century， soot ___。
5. B.分析：结合文章标题(主题)Soot (煤灰) and Snow：a Hot Combination，判断B很可能是答案。也可以借助文章中的相关语句：According to a computer simulation，black soot may be responsible for 25 percent of observed global warming over the past century.
A produces much more global warming than a carbon-dioxide forcing of the same magnitude
C can produce greenhouse gases
D absorb more of sun’s energy and warmth than white background
E still surpass soot in warming the world’s climate during the last century
F can be seen mostly in the Northern Hemisphere
6.Hansen cautioned that greenhouse gases__。
6.E. 分析：文章中提到温室气体仍然是造成全球温度上升的主要因素，因此判断E是答案。
A produces much more global warming than a carbon-dioxide forcing of the same magnitude
C can produce greenhouse gases
D absorb more of sun’s energy and warmth than white background
F can be seen mostly in the Northern Hemisphere
7. Black soot covered snow and ice__。
7. D.分析：该题可以借助常识直接判断D是答案(带有煤灰的冰和雪要比白色的背景吸收的热量多)。
A produces much more global warming than a carbon-dioxide forcing of the same magnitude
C can produce greenhouse gases
F can be seen mostly in the Northern Hemisphere
8. A soot forcing is unusually effective，which__。
8. A.分析：借助提干中的核心词soot forcing作为答案线索，这样发现答案相关句：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(数量)