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  3. Tornadoes, whirlwinds, and watersprouts are rotating funnel cloud air masses of small diameter. They are differentiated by the i

Tornadoes, whirlwinds, and watersprouts are rotating funnel cloud air masses of small diameter. They are differentiated by the i

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问题     Tornadoes, whirlwinds, and watersprouts are rotating funnel cloud air masses of small diameter. They are differentiated by the intensity of their rotation and by the surfaces that they traverse. Though tornadoes and whirlwinds both travel over land masses, whirlwinds are atmospheric systems smaller than tornadoes. Watersprouts are tornadoes that form or pass over a water surface. A tornado is a powerful vortex or "twister" whose rotational speeds are estimated to be near 300 miles per hour. The first visible indication of tornado development is usually a funnel cloud. As this funnel dips earthward, it becomes darker because of the debris that is forced into its intensifying vortex. Some tornadoes give no visible warning Until their destruction strikes the unsuspecting victim. Tornadoes often occur in groups, and several twisters sometimes descend from the same cloud base. The forward speed of a tornado is normally 30 to 40 miles per hour. In the short time that it takes to pass, a tornado can cause fantastic destruction. There have been cases reported in which blades of straw were embeded in fence posts.
The author refers to straw embeded in fence posts in order to show ______.
选项 A、where tornadoes occur
B、who makes reports about tornadoes
C、when tornadoes take form
D、how tornadoes affect physical objects
答案D
解析 这道题要求考生进行分析。前面文中提到龙卷风能造成巨大破坏，然后提到龙卷风使稻草嵌入栅栏柱上的例子，作者的意图当然是要强调龙卷风在物体上作用有多么强大，所以四个选项中很容易找出正确答案。
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