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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.
According to the passage, some tornadoes take people unaware because they ______.
选项 A、produce a sound similar to thunder
B、cannot be seen until it is too late
C、look so much like other clouds
D、carry so many large pieces of debris
答案B
解析 这道题考生注意到文中"Some tornadoes give no visible warning until their distraction strikes the unsuspecting victim．”“no visible”也就是“cannot be seen．”所以应选B。
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