听力原文： One of the most feared occurrences in nature is the tornado. The area most frequ
ently the target of this violent windstorm is the Great Plains, the region extending from the Rockies to the Mississippi, and from Canada down through Texas. When warm, moist air meets with cooler, drier air at low levels, a tornado often occurs. Prior to the formation of the familiar funnel-shaped cloud, the sky is very clear. Then, a black line suddenly appears. As this black area moves in, the fast wind becomes hot and moist and a deep stillness encircles the landscape. Because the area pressure drops steadily during a tornado, breathing becomes difficult. Insects fall to the ground, unable to balance themselves in flight. Suddenly, a black funnel resembling a giant whip dips down out of the sky, destroying what it touches, and then retreats. Although a tornado usually destroys property rather than lives, an average of 120 people die yearly as a result of these violent storms. It is obvious why the tornado is feared throughout the Great Plains.
Prior to the formation of a tornado, ______.
A．the sky is cloudy
B．the sky is very dark
C．the sky is cloudless
D．a black line stretches across the horizon
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