单选题 Passage Two 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 scant. Tundra lands are snow-covered for much of the year, until summer brings a burst of wildflowers. The Arctic tundra, where the average temperature is -12 to -6 degrees Celsius,supports a variety of animal species, including Arctic foxes.polar bears,gray wolven,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 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. 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 decomposes and releases CO, the tundra has flipped from a carbon sink to a carbon contributor. What can we learn about the animal species living in the Arctic tundra