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  3. The opinion of many engineers is that the architects that designed the New York World Trade Center Twin Towers did a good job in

The opinion of many engineers is that the architects that designed the New York World Trade Center Twin Towers did a good job in

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问题     The opinion of many engineers is that the architects that designed the New York World Trade Center Twin Towers did a good job in designing the buildings. The buildings were safe from the hazards envisioned at the time. Indeed, they were designed to withstand the force of aircraft that might crash into them. They were not, however, designed to withstand the effects of the very high temperatures generated by the ignition of tons of aviation fuel carried by the aircraft.
    Steel lightly covered with concrete is the main structural component of the towers and steel loses a lot of structural strength when heated to 1,100 degrees Centigrade and beyond, as it was. To shield the steel the towers beams were coated with a few centimeters of concrete. That insulation was to provide structural integrity for at least 1 hour of fire. The towers met that test. The south tower lasted 62 minutes. The north, 103 minutes. Tragedy followed when not all could escape prior to the collapse.
    Most experts agree that the Twin Towers were very well-designed. Many also agree that the people planning the attacks were very knowledgeable and aimed the aircraft at a height that would effect the most damage. The aircraft crash and the resulting fuel fed fires were aimed at about the 90th floor. That was just high enough to insure that the upper floors would weaken with the heat and crack down, dropping into the remaining part of the building.
    The mass of the falling top floors created such a falling load that some experts estimate it exceeded by 5 to 10 times normal bearing loads. The effect was like a giant hammer, each floor contributing more weight to be born by the next floor down.
    Were the Trade Towers well designed? Engineers say yes. But the public believes that architects will never again design a tall building without considering the effects of terrorist attacks of all potential kinds. That not only includes the ability of the structure to withstand attack but the means to allow the buildings occupants to escape in time to survive.
The part of the building below 90th floor______.
选项 A、was immune to the attack
B、was chosen as the attack aim
C、cracked down because of fire
D、was hammered down by the falling upper part
答案D
解析 本题是推理题。从文中第4段“…the effect was like a giant hammer,each floor contributing more weight to be born by the next floor down．”可以得出，飞机撞至顶层，撞击力像一把锤子，向下面的每层施压下去。由此可以推断：90层以下的楼层是被上层塌下的部分重砸下来的。
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