Avalanche and Its Safety An avalanche is a sudden and rapid flow of snow, often mixed with air and water, down a mountainsid

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问题                         Avalanche and Its Safety
    An avalanche is a sudden and rapid flow of snow, often mixed with air and water, down a mountainside. Avalanches are【C1】______the biggest dangers in the mountains for both life and property.
    All avalanches are caused by an over-burden of material, typically snowpack, that is too massive and unstable for the slope【C2】______supports it. Determining the critical load, the amount of over-burden which is【C3】______to cause an avalanche, 【C4】______a complex task involving the evaluation of a number of factors.
    Terrain slopes flatter than 25 degrees or steeper than 60 degrees typically have a low 【C5】______of avalanche. Snow does not【C6】______significantly on steep slopes; also, snow does not【C7】______easily on flat slopes. Human-triggered avalanches have the greatest incidence when the snow’s angle of rest is【C8】______35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. The rule of thumb is: A slope that is【C9】______enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle. Additionally, avalanche risk increases with 【C10】______; that is, the more a slope is disturbed by skiers, the more likely it is that an avalanche will occur.
    Due to the complexity of the subject, winter travelling in the backcountry is never 100% safe. Good avalanche safety is a continuous【C11】______, including route selection and examination of the snowpack, weather【C12】______, and human factors. Several well-known good habits can also 【C13】______the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings should be paid 【C14】______to. Never follow in the tracks of others without your own evaluations; snow conditions are almost certain to have changed since they were made. Observe the terrain and note obvious avalanche paths where plants are 【C15】______or damaged. Avoid travelling below others who might trigger an avalanche.
【C4】

选项 A、are
B、will be
C、is
D、was

答案C

解析 该句的主语是Determining the critical load,从上下文来看应该使用一般现在时,所以C是正确答案。
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