An avalanche is a sudden and rapid flow of snow, often mixed with air and water, down a mountainside. Avalanches are 1 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 2 supports it. Determining the critical load, the amount of over-burden which is 3 to cause an avalanche, 4 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 low5 of avalanche. Snow does not6 significantly on steep slopes; also, snow does not7 easily on fiat slopes. Human-triggered avalanches have the greatest incidence when the snow's angle of rest 1is 8 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. The rule of thumb2 is: A slope that is 9 enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle. Additionally3, avalanche risk increases with 10 ;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 backcountry4 is never 100% safe. Good avalanche safety is a continuous 11 , including route selection and examination to the snowpack, weather 12 , and human factors. Several well-known good habits can also 13 the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings should be paid 14 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 15 or damaged. Avoid traveling below others who might trigger an avalanche.
练习:
1. A among B of C to D in
2. A when B that C who D whose
3. A mostly B likely C clearly D surely
4. A are B will be C is D was
5. A weight B form C risk D work
6. A fall B flow C roll D gather
7. A fall B flow C roll D gather
8. A among B between C with D for
9. A thick B thin C flat D rocky
10. A use B time C snow D rain
11. A journey B trip C fact D process
12. A conditions B reports C forecast D event
13. A increase B reduce C improve D remove
14. A price B effort C attention D money
15. A missing B grown C big D fresh
答案与题解：
1.A表达“雪崩是山上可能发生的最大危险之一”的意思，因此应该选择among(在……之中)。
2.B从该句的语法结构上来看，此处需要一个关系代词，代替slope，所以that是最佳选择。选项A、C、D均不符合语法。
3.B选项A不符合语法，c和D符合语法，但不符合常识：过度的雪的积压可能导致雪崩，而不是必定导致雪崩。所以，B是最佳答案。
4.C该句的主语是Determining the critical load，从上下文来看应该使用一般现在时，所以C是正确答案。
5.C要确定本题答案的一个有效的方法是排除法。a.10w weight/form work of avalanche都不合逻辑，只有a low risk of avalanche符合上下文的意思。下面的句子解释了low risk of avalanche的道理，更证实了选risk是正确的。
6.D第6和第7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。
7.B(见6)
8.B理解了句子的意思就不难判断选项：雪在静止状态下，角度在35_45度之间，最可能发生人为触发的雪崩。between：在……之间。
9.C这个句子说明的是什么样的山坡最易发生雪崩，即，A slope that is flat enough to hold snow but steep enough to ski。flat在此做“平坦”解，与后面的steep形成反义。
10.A句中的that is表明，后半部分是对前半部分的进一步说明。所以，这里的选择要根据下文的意思判断。use是最佳选择，整个句子的意思是：山坡被滑雪者使用的越多，雪崩就越可能发生。
11.D尽管选项A、B和D都能和continuous搭配，从全段的内容判断，只有process是最佳选择，因为该段描写的是如何防备雪崩，及如何做好安全措施等一系列问题。
12.A选项A、B、c都可以与weather搭配，但是根据上下文，只有A最为符合文章的内容。
13.B选项A不符合句子的意思;c不能和risk搭配;D也不符合句子的意思，因为不可能完全消除雪崩的隐患。
14.C该句主句使用的是被动语态，第二个动词是pay attention t0的被动形式。l选项A、B、D均不符合句子的意思。
15.A该句是作者给出的一系列忠告之一，即，认真观察地形，注意明显的雪崩路径：没有植物或植物被毁坏的地方。选项8、C、D均不符合句义。