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  3. Tundra ecosystems are treeless regions found in the Arctic and on the tops of mountains, where the climate is cold and windy and

Tundra ecosystems are treeless regions found in the Arctic and on the tops of mountains, where the climate is cold and windy and

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问题     Tundra ecosystems are treeless regions found in the Arctic and on the tops of mountains, where the climate is cold and windy and rainfall is low. Tundra lands are snow-covered for most of the year, until summer brings a burst of wild flowers.
    The Arctic tundra, where the average temperature is rather low, supports a variety of animal species, including Arctic foxes, polar bears, Arctic wolves, caribou, snow geese and musk oxen. The summer growing season is just 50 to 60 days, when the sun shines 24 hours a day.
    The few plants and animals that live in the harsh conditions of the tundra are essentially clinging to life. They are highly vulnerable to environmental stresses like reduced snow cover and warmer temperatures brought on by global warming.
    The Arctic tundra is changing dramatically due to global warming. Already, more southern animals like the red fox have moved onto the tundra. The red fox is now competing with the Arctic fox for food and territory.
    Another major concern is that the melting of the permafrost (永冻土层) is contributing to global warming. Estimates suggest that about 14 percent of the Earth’s carbon is tied up in the permafrost. Until recently, the tundra acted as a carbon sink and captured huge amounts of carbon dioxide from the atmosphere as part of photosynthesis(光合作用). This process helped keep the amount of this greenhouse gas from accumulating in the atmosphere. Today, however, as the permafrost melts and dead plant material rots(腐烂) and releases CO2, the tundra has changed from a carbon sink to a carbon contributor.
The tundra used to be a carbon sink because ________.
选项 A、it supported a variety of animal species
B、it captured huge amounts of carbon dioxide from the atmosphere
C、the temperature was extremely low there
D、the tundra ecosystems were treeless regions
答案B
解析 根据最后一段第三句“…the tundra acted as a carbon sink and captured huge amounts of carbon dioxide from the atmosphere…”可知，苔原在过去充当着碳汇的作用，从大气中捕获大量的二氧化碳。故选B。
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