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  3. The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot intense mass of gas calle

The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot intense mass of gas calle

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问题     The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot intense mass of gas called a fireball. For a l0-kiloton (千吨，核弹当量单位) explosion in the air, the fireball will attain a maximum diameter of about 300m; for a l0-megaton (兆吨，核弹当量单位) weapon the fireball may be 4.8 km across. A flash of heat radiation is emitted from the fireball and spreads out over a large area, but with steadily decreasing intensity. The amount of heat energy received a certain distance from the nuclear explosion depends on the power of the weapon and the state of the atmosphere. If the visibility is poor or the explosion takes place above clouds, the effectiveness of the heat flash is decreased. The heat radiation falling on exposed skin can cause what called flash burns. A 10-kiloton explosion in the air can produce moderate (second-degree) flash burns, which require some medical attention, as far as 2.4 km from ground zero; for a 10-megaton bomb, the corresponding distance would be more than 32 km. milder burns of bare skin would be experienced even farther out. Most ordinary clothing provides protection from the heat radiation, as does almost any opaque (不透明的) object. Flash burns occur only when the bare skin is directly exposed, or if the clothing is too thin to absorb the heat radiation.
    The heat radiation can start fires in dry, flammable materials, for example, paper and some fabrics, and such fires may spread if conditions are suitable. The evidence from the A-bomb explosions over Japan indicates that many fires, especially in the area near ground zero, originated from secondary causes, such as electrical short circuits, broken gas lines, and upset furnaces and boilers in industrial plants. The blast damage produced wreckage that helped to maintain the fires and denied access to fire-fighting equipment. Thus, much of the fire damage in Japan was a secondary effect to the blast wave.
    Under some conditions, such as existed at Hiroshima but not at Nagasaki, many individual fires can combine to produce a fire storm similar to those that accompany some large forest fires. The heat of the fire causes a strong updraft (向上排气), which produces strong winds drawn in toward the center of the burning area. These winds fan the flame and convert the area into a disaster in which everything flammable is destroyed.
It can be inferred that amount of radiation______.
选项 A、is the greatest when atomic bomb is dropped from the air
B、is the weakest when it spreads over a large area
C、is the greatest when the atomic bomb goes off on the ground
D、becomes increasingly weak as it travels far out
答案D
解析 语义理解题。该句说“火球会快速释放出可以覆盖大片区域的热辐射，不过其强度会逐渐下降。”所以，热辐射发散的距离越远，强度就越小。所以，D“随着穿过的距离而变得越来越弱”正确。
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