第六部分：完型填空(第51～65題，每題1分，共15分)
閱讀下面的短文，文中有15處空白，每處空白給出了4個選項，請根據短文的內容從4個選項中選擇1個最佳答案，涂在答題卡相應的位置上。
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 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.
51 A among
B of
C to
D in
52 A when
B that
C who
D whose
53 A mostly
B likely
C clearly
D surely
54 A are
B will be
C is
D was
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
