题目内容：
回答题：Avalanche and Its Safety
An avalanche is a sudden and rapid flow of snow, often ________________________ (51) with air and water, down a mountainside．Avalanches are among the biggest dangers in the mountains for both life and property.
All avalanches are ________________________ (52) by an over-burden of material, typically snowpack that is too massive and unstable for the slope that supports it．Determining the critical load, the________________________ (53) of over-burden which is likely to cause an avalanche, is a complex task involving the evaluation of a number of factors.
Terrain slopes flatter than 25 degrees or steeper than 60 degrees ________________________ (54) 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.
A．mixed
B．mixing
C．mixes
D．have mixed
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