Force other than damaging winds are also at work inside tornadoes. Sometimes, as the turning, jumping funnel(漏斗) passes over a house, the walls and ceiling burst apart as if a bomb had gone off inside. This explosion(爆炸) 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 funnel passes over a small building that measures 20 by 10 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 great inside pressure may cause the building to burst.

　　Unfortunately, heavy rain happens in thunderstorms that later produce tornadoes. So, people often shut all windows to protect their property. This may cause far worse damage later.

63. Which of the following is the main topic of the passage?

A. Where tornadoes are formed. 　　      B. When tornadoes usually occur.

C. How tornadoes can be prevented.　　     D. Why tornadoes cause so much damage.

64. According to the passage, tornadoes can destroy building because the _____.

A. force of a tornado increases the air pressure in a building

B. air pressure at the center of a tornado is over 172,000 pounds

C. weight of a tornado can crush a building’s roof when it passes overhead

D. air pressure inside a tornado is less than the air pressure inside a building

65. According to the passage, the pressure on a building during a tornado can be reduced　　　　by _____.

A. closing the windows 　　　　　        B. opening the windows

C. using an electric fan 　　　　          D. making the roof and walls stronger