GRE考试

【单选题】Air turbulence can substantially accelerate the appearance of large dropletstriggering rain by presenting a new mechanism, the "sling effect", which increases collisions of droplets that have become detached from the airflow. First, vapor condensation in cloud cores produces small droplets resembling one another in size, which then expand to raindrop size by coalescing under the effects of air turbulence―a force thought to cause collisions of similar-sized droplets whose radii exceed a few micrometers. Then, turbulent vortices act as small centrifuges that spin heavy droplets out, creating concentration heterogeneities and jets of droplets, both of which increase the mean collision rate, which in turn accelerates rain initiation.　　One can conclude that rain prediction requires a quantitative description of droplet collision in turbulence, a mechanism which helps meteorologists to forecast rainfall, but detailed understanding of the phenomenon entails consideration of such factors as warm and cold fronts stretching over hundreds of miles, individual clouds perhaps a mile or so across, and even, as the mechanism illustrates, tiny eddies perhaps a few centimeters or so in size. According to the passage, air turbulence generally does which of the following Ⅰ.Creates small droplets of equal size to form in cloud cores.Ⅱ.Causes different concentrations of rain droplets to form in different places.Ⅲ. Speeds up the rate by which large droplets appear.
A、Ⅰ only
B．Ⅱ only
C．Ⅰ and Ⅱ only
D．Ⅱ and Ⅲ only
E．Ⅰ, Ⅱ, and Ⅲ

网考网参考答案：D
网考网解析：
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