单项选择题

案例分析题Today’s lecture is about two powerful natural forces, one destructive, yet the other productive.
Tornado (龙卷风) is a typical example of destructive forces in nature. Forces other than damaging winds are also at work inside tornadoes. Sometimes as twisting, screaming wind passes over house, the walls and the ceiling burst apart as if a bomb had gone off inside. This explosion is caused by the low air pressure at the center of a tornado.
The pressure at the center of a tornado is usually 13 pounds per square inch. However, inside the house the air pressure is normal, about 15 pounds per square inch. The difference of 2 pounds per square inch between the inside and outside pressure may not seem like much. But suppose a tornado passes over a small building that measures 20 by 10 feet. On each square inch of the building, there is 2 pounds of pressure from the inside that is not balanced by air pressure outside the building. On the ceiling, that adds up to an unbalanced pressure of 57,600 pounds. The pressure on the four walls adds up to 172,800 pounds.
If windows are open in the building, some of the inside air will rush out through them. This will balance the pressure inside and outside the building. But if the windows are shut tightly, the enormous inside pressure may cause the building to burst. Unfortunately, heavy rain and hail often occur in thunderstorms that later produce tornadoes. So people frequently shut all windows to protect their property, which may cause far worse damage later. For the same reason, tornado cellars must have an air opening. Otherwise, the cellar door might be blown out when a tornado passes over it.
Even more amazing is the strong force of plants in water evaporation (蒸发). The task of carrying 40 gallons of water up almost 60 feet—to an apartment on the sixth floor, for instance—is an undeniably heavy task. And yet it is no more than a full-grown tree does on any warm day. For 400 trees 75 to 90 feet high, the leaves evaporate an average of 20 tons of water on a single day—equivalent to the capacity of a larger tanker truck. Before this water evaporates it is lifted an average of 80 feet through trunks and branches. Anyone thinks this a trifle would do well to work out how many bucks it would mean, to which floor. Quite an achievement. The surprising thing is that the plants themselves need supply no energy to do this. Everything happens automatically. The evaporation from the leaf surface causes a constant compensatory suction (吸) of water. This suction communicates itself through branch and trunk down into roots. The suction force caused by evaporation from the tiny openings of the leaves is tremendously strong. When, on a dry summer’s day, the surrounding atmosphere has a relative humidity (湿度) of 45 percent, the evaporation suction corresponds to the pull exerted on a rope 3 mm in diameter by a 154-pound man suspended from it! Thus evaporation actually draws the water from the leaves by force, and pulls more water up to replace it. The driving force is the sun and plants make direct use of its energy for water transport. Once again, we see the powerful, unlimited energy in nature.
People often close their windows to prevent damage caused by （）.

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