题目内容：
根据以下内容，回答题。Passage Four
Climate, more than any other single factor, determines the distribution of life on earth. Climatic boundaries establish the limits which organisms can survive. Plants, even more than animals, must be well adapted to climate in order to survive. They cannot move about or take shelter but must be equipped to endure whatever weather conditions are likely to occur. In the harsh conditions of the tundra, for example, low growing mosses, lichens, and a few flowering plants all hug the ground for shelter from icy winds.
Animals, despite their ability to move about and find shelter, are just as much influenced by climate as plants are. Creatures such as the camel and the penguin are so highly specialized that they have an extremely limited distribution Others, such as bears are flexible enough to adapt to a broad range of climates. Oceandwelling organisms are just as sensitive to climatic changes--in this case temperature and salinity--as land animals. Reef corals can survive only in clear warm seawater. Certain foraminifers are so sensitive to changes in their environment that their presence can be taken as an index of sea temperature. Human beings are among the
least specialized of all animals and can live almost anywhere. Their clothes and their homes act as a sort of "miniature climate" that can be taken with them everywhere.
According to the passage, plants on the tundra grow in the ground__________. A．to avoid being eaten by arctic animals
B．because fertilizer is not readily available
C．to minimize exposure to the cold
D．because unfrozen water supplies are very scarce
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