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  3. 1 Permafrost occurs in areas where the mean annual temperature is at or below minus nine degrees Celsius. In Canada, where over

1 Permafrost occurs in areas where the mean annual temperature is at or below minus nine degrees Celsius. In Canada, where over

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问题 1 Permafrost occurs in areas where the mean annual temperature is at or below minus nine degrees Celsius. In Canada, where over fifty percent of the land area is permanently frozen, the depth of the permafrost ranges from two meters at the southern edge to as many as 300 meters in the far north. Almost all of the soil moisture occurs in the form of ground ice, which creates a challenge for the plants and animals that must survive in the North.
2 Temperature changes take a long time to penetrate the ground, and deep layers that froze during the last Ice Age remain insulated from current surface conditions. A thin surface layer may thaw if air temperatures rise above freezing in the summer. However, a brief thaw promotes the growth of sparse vegetation, which acts as an insulating layer for the lower levels, keeping them frozen.
Why do the deep layers of ground remain frozen even when temperatures rise above freezing?
选项 A、Small variations in temperature do not affect surface conditions.
B、Climate change in the North is contributing to a new Ice Age.
C、The ground thaws only when the temperature rises and falls rapidly.
D、Vegetation insulates the deep layers from surface temperatures.
答案D
解析 The deep layers of ground remain frozen because vegetation insulates them from surface temperatures. Clues: ...deep layers...remain insulated from current surface conditions; ...sparse vegetation, which acts as an insulating layer for the lower levels, keeping them frozen.
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