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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.
The term permafrost in paragraph 1 is used to describe
选项 A、a very cold temperature range
B、ground that remains frozen
C、water that is deep underground
D、organic matter in the soil
答案B
解析 Permafrost describes ground that remains frozen. Clues: ...occurs in areas where the mean annual temperature is at or below minus nine degrees Celsius; ...where over fifty percent of the land area is permanently frozen...; . ..ground ice....
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