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  3. Lichens, probably the hardiest of all plants, live where virtually nothing else can—not just on rugged mountain peaks but also o

Lichens, probably the hardiest of all plants, live where virtually nothing else can—not just on rugged mountain peaks but also o

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问题     Lichens, probably the hardiest of all plants, live where virtually nothing else can—not just on rugged mountain peaks but also on sunbaked desert rocks. They are usually the first life to appear on a mountainside that has been scraped bare by an avalanche.
    Unlike other members of the plant kingdom, lichens are actually a partnership between two plants. The framework of a lichen is usually a network of minute hairlike fungus that anchors the plant. The other component is an alga, similar to the green film of plant life that grows on stagnant pools, that is distributed throughout the fungus. Being green plants, algae are capable of photosynthesis—that is, using energy from the Sun to manufacture their own food. The fungi are believed to supply water, minerals, and physical support to the partnership.
    Lichens are famous for their ability to survive water shortage. When water is scarce(as is often the case on a mountain), lichens may become dormant and remain in that condition for prolonged periods of time. Some lichens can even grow where there is no rain at all, surviving on only occasional dew—the moisture that condenses on the surface of the plants at night, And unlike most other plants, lichens are little affected by the strong ultraviolet rays in the mountains.
    Lichens use little energy, for they grow slowly. Some grow so slowly and are so old that they are called "time stains." You may find lichens that are centuries old: certain of these lichen colonies have been established for an estimated 2,000 years.
    For decades, scientists wondered how the offspring of an alga and a fungus got together to form a new lichen, it seemed unlikely that they would just happen to encounter one another. It was finally discovered that in many cases the two partners have never been separated. Stalklike "buds" that form on certain lichens are broken off by the wind or by animals: these toll or are blown to a new location.
It can be inferred from the passage that lichens use less energy and grow more slowly when______.
选项 A、the environment is polluted
B、they are exposed to ultraviolet rays
C、they are very old
D、the supply of water is inadequate
答案D
解析
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