题目内容：
根据下列短文，回答题。Avalanche and Its SafetyAn avalanche is a sudden and rapid flow of snow, often mixed with air and water, down a mountainside. Avalanches are 51 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 52 supports it. Determining the critical load, the amount of over-burden which is 53 to cause an avalanche, 54 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 55 of avalanche. Snow does not 56 significantly on steep slopes；also, snow does not 57 easily on flat slopes. Human-triggered avalanches have the greatest incidence when the snow's angle of rest is 58 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 59 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 60；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 61 ， including route selection and examination of the snowpack, weather 62 ，and human factors. Several well-known good habits can also 63 the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings should be paid 64 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 65 or damaged. Avoid traveling below others who might trigger an avalanche. 文章大意：雪崩是雪掺杂着空气和沿着山体突然迅猛地滑动造成的。雪崩是造成山区人民生命和财产安全的最大危险之一。认真观察地形，注意明显的雪崩路径：没有植物或植物被毁坏的地方。不要在那些可能引发雪崩的人或事物下面行走。回答第51空。 A．A among
B．B of
C．C to
D．D in
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