第6部分：完形填空。
下面的短文有15处空白，请根据短文内容为每处空白确定1个最佳选项。
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.
51. A. mixed B. mixing C. mixes D. have mixed
52. A. increased B. caused C. removed D. encountered
54. A. clearly B. likely C. typically D. surely
53. A. deal B. amount C. number D. much
55. A. weight B. form C. risk D. work
56. A. fall B. flow C. roll D. gather
57. A. fall B. flow C. roll D. gather
58. A. among B. between C. with D. for
59. A. thick B. thin C. flat D. rocky
60. A. use B. time C. snow D. rain
61. A. journey B. trip C. fact D. process
62. A. conditions B. reports C. forecast D. event
63. A. increase B. reduce C. improve D. remove
64. A. price B. effort C. attention D. money
65. A. missing B. grown C. big D. fresh
参考答案：
51-55 ABBCC
55-60 DBBCA
61-65 DABCA
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