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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.
The purpose of mentioning the Weddell Sea is to ______.
选项 A、give an example of cold water salinity
B、point out the location of deep waters
C、compare Antarctic waters with Arctic waters
D、indicate that this body of .water has salinity variations
答案A
解析 本文最后一段中提到Weddell海是为了举例说明冷水含盐量。文中提到Weddell海是在叙述了第三种改变含盐量的过程之后，这一种改变主要与冰的形成与融化有关。海水结成冰后含盐量升高，冰融化后含盐量又变低。随后，文中以weddell海为例，说明海水结冰后，冰冷的海水含盐量增高。所以，这一题显然应该选A项。B项指出渗水的位置。C项对比南极洲水域和北冰洋。D项表示不同水域含盐量不同。
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