Forces other than damaging winds are also at work inside tornadoes(龙卷风). Sometimes, as the wind passes a house, the walls and 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 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 57600 pounds. The pressure of the four walls adds up to 172800 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 huge inside pressure may cause the building to burst.

　　Unfortunately, heavy rain often occurs in the storms that later produce tornadoes, so people often shut their windows. This may cause far worse damage later.

　　1.What did the paragraph before this passage most probably discuss?

　　A. Measuring rainfall from a storm

　　B. The powerful winds of tornadoes.

　　C. The kinds of damage caused by explosions

　　D. Repairing the damage from tornadoes.

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

　　A. How tornadoes can be prevented.

　　B. When tornadoes usually occur.

　　C. Where tornadoes are formed.

　　D. Why tornadoes cause so much damage.

　　3.Tornadoes can destroy buildings 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 172000 pounds.

　　C. weight of a tornado can destroy a building‘s roof when it passes over head.

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

　　4.What is the difference per square inch between the air pressure inside a building and the air pressure inside a tornado?

　　A. 2 pounds. B. 10 pounds. C. 13 pounds. D. 15 pounds.

　　答案：CDDA