Passage 4

　　When we analyze the salt salinity(鹽濃度) of ocean waters, we find that it varies 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. In this manner, the salinity is increased, since the salt stays behind. If this is carried to the extreme, of course, white 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, such as rain, by which water is added to the ocean. Here the ocean is being diluted (稀釋) 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.

　　Normally, in hot 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 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, the densest water in the ocean 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 portion of the oceans of the world.

　　26. This passage mainly tells us about_______.

　　A. the analysis of the salinity of ocean waters

　　B. the causes of the variation in oceanic salinity

　　C. the importance of the changes in oceanic salinity

　　D. the different forms of salts in ocean waters

　　27.It can be known from the passage that increase in the salinity of ocean water is caused

　　by______.

　　A. melting of sea ice B. precipitation

　　C. evaporation D. supplement of salt

　　28.Which of the following is not the cause of the decrease in the oceanic salinity?

　　A. Precipitation. B. Rain and snow.

　　C. Formation of ice. D. Addition of water by rivers.

　　29.Which of the following statements is TRUE according to the passage?

　　A. Oceanic salinity has great effect on sea life.

　　B. Many factors combine to cause changes in oceanic salinity.

　　C. The movement of the water is related to the amount of salt.

　　D. The temperature of the water has much to do with the oceanic salinity.

　　30.The purpose of mentioning the Weddell Sea is_______.

　　A. to give an example of cold-water salinity

　　B. to point out the location of deep waters

　　C. to make a comparison between hot-water salinity and cold-water salinity

　　D. to show that the water in the Weddell Sea varies in salinity from place to place

　　II. C 12. L 13. O 14. D 15. F 16. G 17. N 18. A 19. J 20. K 21. A 22. C 23. A 24. B 25. A 26. B 27. C 28. C 29. D 30. A