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  3. If the salinity (含盐量) of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of

If the salinity (含盐量) of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of

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问题     If the salinity (含盐量) of ocean waters 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; this, by the way, is how much of the table salt we use is actually obtained.
    The opposite of evaporation is precipitation (降水), such as rain, by which water is added to the ocean. Here the ocean is being diluted(冲淡)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 substraction of the 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 in other oceanic areas.
    A third process by which salinity may be altered is associated with the formation and melting of sea ice. When seawater is frozen, the dissolved(溶解) materials are left behind. In this manner, seawater 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, of America, 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.
It can be known from the passage that increase in the salinity of ocean water is caused by______.
选项 A、melting of sea ice
B、precipitation
C、evaporation
D、supplement of salt
答案C
解析 文中第一段第四句、第五句介绍了影响含盐量的因素：蒸发(evaporation)，蒸发使含盐量增加，所以，答案为C选项。
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