单项选择题

According to the passage, scientists have decided that sulfur aerosols from volcanic eruptions affect global temperatures to a greater extent than ash clouds because

A．the greenhouse gas effect produced by human activities tends to have a much greater environmental effect than that of volcanic explosions
B．clouds of sulfur-rich gases tend to achieve greater heights in the stratosphere than do ash clouds
C．a particular explosion producing high sulfur and little ash had a greater environmental effect than the converse
D．the burning of fossil fuels tends to exacerbate the effects of sulfurous aerosols, but not those of ash clouds
E． global warming effects tend to minimize the impact of ash clouds, but not those of sulfurous aerosols

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单项选择题

Which of the following, if presented as the first sentence of a succeeding paragraph, would most logically continue the discussion presented in the passage

A．The influence of volcanic eruptions on global weather patterns is almost completely understood at present.
B．Scientists have only begun to understand, however, the effects of these wavelengths of light on the global environment.
C．The majority of global cooling effects have thus been attributed to volcanic explosions.
D．The human warming activities ameliorate by volcanic eruptions include those produced both by heavy industry and agriculture.
E． The reason that sulfate aerosols take an unusually large time to disperse owes primarily to meteorological factors, including the jet stream.

单项选择题

Which of the following best describes the organization of the third paragraph of the passage

A．A theory is proposed, considered, and then amended.
B．Opposing views are presented, elaborated, and then reconciled.
C．A problem is described, then a solution is discussed and its effectiveness is affirmed.
D．A view is advanced, then refuted, and an alternative is suggested.
E． A hypothesis is presented, qualified, and then reaffirmed.