托福考试

【单选题】
{$mediaurl} What is the lecture mainly about
A、The role of permafrost inArctic environments.
B、The characteristics and classification of permafrost.
C、The natural process by which permafrost forms.
D、Two main categories of permafrost.
网考网参考答案：B
网考网解析：
[解析] 12-17 [Professor] (mate) Q12 Today, we’ll be continuing with our theme of Arctic environments by discussing permafrost. I suppose you’ve come across this term before in your Earth science classes...but I’ll provide a quick definition anyway. Permafrost is ground--like rock or soil--that remains below zero degrees Celsius, or at zero, for at least two years. Zero degrees is, of course, the freezing point of water. Quite simply, the designation "permafrost" is based only on temperature. Naturally, permafrost is commonly found in the higher latitudes, um, the ones nearer to the North and South Poles--places like Greenland, for example, which stretches all the way to 84 degrees north. But elsewhere in the Northern Hemisphere, um, most permafrost lies between about 60 and 68 degrees north. See, above that, it’s mainly the Arctic Ocean--no land. Q13 However, there’s also such a thing as "alpine" permafrost too, which can be present at much lower latitudes because it’s found at high altitudes, where the temperature is colder. Um, for example, this happens in the Himalayas, which are at a relatively low latitude. All totaled, permafrost accounts for about 24% of the planet’s landmass. Um, that’s about 22.79 million square kilometers of Earth’s exposed land. Q15(Q)Q15(C) In many areas, permafrost is covered by a thin layer of soil that’s subject to thawing during warmer months and refreezing during the winter. This layer is known as the active layer. It’s usually about 0.6 to 4 meters thick; the depth depends a lot on the region and the conditions there. The characteristics of the permafrost layer itself can vary a little bit. Sometimes--if it’s composed of a nonporous material like bedrock--it won’t contain any ice at all. In other cases, it’ll contain up to 30% ice. Sometimes, permafrost may be covered by snow...and sometimes it isn’t. There’s a huge range of thickness, too. It may be just a meter in thickness, but at its thickest, permafrost can be 1,000 meters deep. Q16 Interestingly enough, even though it’s relatively easy to classify permafrost -the ground surface has to be at zero degrees or less for two years, remember--it’s not so easy to exactly map its distribution. Common sense might tell us that permafrost should be found in places where the air temperature remains below freezing for most of the year, and that’s generally true...but, as it turns out, it’s not entirely that simple. Permafrost doesn’t always exist where air temperatures remain below freezing. Even beneath glaciers, where the air temperature is certainly very cold, there’s no guarantee that there’ll be permafrost. Q12 Permafrost can be divided into two main categories. First, there’s discontinuous permafrost. This is the kind of permafrost you’d find in a region where the temperature barely stays below freezing throughout the year. If the annual surface temperature averages between 0 and -5 degrees Celsius, permafrost is only intermittent. It’s discontinuous. Now, since this makes for a broad category--I mean, in some places you could have just tiny little patches of permafrost, with most of the land unfrozen...and in other places there might be mainly permafrost with tiny patches of unfrozen ground. Q14 To account for these differences, the category of discontinuous permafrost is further divided into "extensive" discontinuous permafrost--where permafrost covers between half and ninety percent of the land--and "sporadic" permafrost...where permafrost covers less than half of the landscape. Um, the extensive discontinuous permafrost zone generally has temperatures that average between -2 degrees Celsius and -4 degrees Celsius. Q15(E) Sporadic permafrost makes up the warmer zones of discontinuous permafrost, where average temperatures are between zero and -2 degrees Celsius. Q17 The other main category of permafrost...um, as o 查看试题解析出处>>