【单选题】
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。
An avalanche is a sudden and rapid flow of snow, often mixed with air and water, down a mountainside.Avalanches are{{U}} (51) {{/U}}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{{U}} (52) {{/U}}supports it.Determining the critical load, the amount of over-burden which is{{U}} (53) {{/U}}to cause an avalanche,{{U}} (54) {{/U}}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{{U}} (55) {{/U}}of avalanche. Snow does not{{U}} (56) {{/U}}significantly on steep slopes; also, snow does not{{U}} (57) {{/U}}easily on flat slopes. Human-triggered avalanches have the greatest incidence when the snow’s angle of rest is{{U}} (58) {{/U}}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{{U}} (59) {{/U}}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{{U}} (60) {{/U}}; that is, the more a slope is disturbed by skiers, the more likely it is that an avalanche will occur.
Duc to the complexity of the subject, winter travelling in the backcountry is never 100% safe. Good avalanche safety is a continuous{{U}} (61) {{/U}}, including route selection and examination of the snowpack, weather{{U}} (62) {{/U}}, and human factors. Several well-known good habits can also{{U}} (63) {{/U}}the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings should be paid{{U}} (64) {{/U}}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{{U}} (65) {{/U}}or damageD、Avoid traveling below others who might trigger an avalanche.
B．flow
C．roll
D．gather
网考网参考答案：D
网考网解析：
第56和第57题可以一起考虑。整个句子的意思是：在坡度大的坡上，雪不会大量堆积。在较为平坦的坡面，雪不会轻易滑动。 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>
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