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  3. The Salinity of Ocean Water If the salinity of ocean water is analyzed, it is found to vary only slightly from place to plac

The Salinity of Ocean Water If the salinity of ocean water is analyzed, it is found to vary only slightly from place to plac

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问题                         The Salinity of Ocean Water
    If the salinity of ocean water is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity.
    One of these is the subtraction of water from the ocean by means of evaporation—conversion of liquid water to water vapor. In this manner the salinity is increased, since the salts stay behind. If this is carried to the extreme, of course, white crystals of salt would be left behind.
    The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.
    Normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than that in other oceanic areas.
    A third process by which salinity may be altered is associated with the formation and melting of sea ice. When sea water is frozen, the dissolved materials are left behind. In this manner, sea water directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water. In the Weddell Sea Antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.
What does the passage mainly discuss?
选项 A、The elements of salt.
B、The bodies of water of the world.
C、The many forms of ocean life.
D、The salinity of ocean water.
答案D
解析 主旨大意题。结合文章的标题，通读全文会发现，整篇文章都在说海水含盐量的变化。因此选择D。
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