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。

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 (51) the biggest ers 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. Duc 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 () 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.