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  3. Tornadoes, whirlwinds, and water-sprouts are rotating funnel cloud air masses of small diameters. They are differentiated by the

Tornadoes, whirlwinds, and water-sprouts are rotating funnel cloud air masses of small diameters. They are differentiated by the

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问题      Tornadoes, whirlwinds, and water-sprouts are rotating funnel cloud air masses of small diameters. They are differentiated by the intensities 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. Water-sprouts 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.
Why do tornadoes appear to be dark?
选项 A、They occur only at night.
B、They are funnel-shaped.
C、They pick up debris from the ground.
D、They are water-laden.
答案C
解析 由文中As this funnel dips earthward，it becomes darker because of the debris that is forced into its intensifying vortex可知。
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