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  3. Few natural dangers are more feared than avalanches. Avalanches are a familiar part of European history. Particularly in the Swi

Few natural dangers are more feared than avalanches. Avalanches are a familiar part of European history. Particularly in the Swi

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问题     Few natural dangers are more feared than avalanches. Avalanches are a familiar part of European history. Particularly in the Swiss and French Alps. This is where the direction of wars has turned almost instantly because of avalanches wiping out invading armies.
    In North America, avalanches are limited almost entirely to the Rocky Mountains and the lower ranges to the west, the Sierra Nevadas and the Cascades. Avalanches have occurred in the mountains of New England but not with the regularity and intensity seen in the western mountains.
    Several methods are used in explaining and predicting avalanches. Scientists are learning about them using research methods. So many of the factors that create avalanches are hidden beneath the snow’s surface that predictions are still largely guesswork. Therefore, winter travelers must assume the worst of conditions when they traverse the slopes.
    An avalanche occurs when a given amount of snow becomes too heavy for whatever is holding it in place. It then breaks loose and slides downhill.
    Avalanches are divided into two general categories, loose snow and slab. A loose snow avalanche usually starts at a single point, such as a skier’s track, and spreads out like a fan or a pyramid in a chain reaction. One crystal breaks another free, which multiples as the loose snow moves downhill.
    Sometimes these avalanches stop after only a few feet. Sometimes they move thousands of tons of snow downhill in speeds up to 300 miles per hour. This creates a shock wave that can flatten parts of a forest that are not even touched by the actual avalanche.
    Stab avalanches are those that have a wide area of snow which breaks loose in a large piece. These can range in size from just a few square feel to thousands of square feet of snow. The most dangerous and common type of avalanche for skiers is the so-called "soft slab" avalanche. This type occurs most often during, or just after a heavy snowfall. The snow hasn’t yet had a chance to settle and adhere to the existing snow. The heavier and the wetter the snow and the colder the temperature, the less likely the new snow will form a bond with the existing snow.
According to the passage, which factor causes an avalanche?
选项 A、The slope of the mountain.
B、The size of the snowfall.
C、The amount and intensity of movement around the snowfall.
D、The weight of the snow.
答案D
解析 细节概括题。根据题干关键词factor causes an avalanche定位到原文第四段。原句中的amount ofsnow becomes too heavy与选项D对应，当某个地方的积雪因为太重而发生倒塌，雪堆下滑就形成雪崩。故答案为D。
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