If the salinity(含鹽量) 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 staybehind. If this is carried to the extreme, of course, white crystals of salt would be left behind.
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(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 or runoff(形成地表水部分的降水).
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 whererivers 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, off Antarctica, the densest water in the oceans is formed as a result of this freezingprocess, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.
練習(xí)題：
Choose correct answers to the question:
1.The best title of the passage can be “_____”.
A.Typical Oceans and Their Respective Features
B.The Causes of the Changes in Salinity of Ocean Water
C.Different Oceans Have Different Salinity
D.The Precipitation and Evaporation of Oceans
2.Which of the following processes will increase salinity of ocean waters?
A.Evaporation.
B. Precipitation.
C. Melting.
D. Dilution.
3.According to this passage, the sea _ is likely to have the lowest salinity.
A. in tropical areas
B. off Antarctica
C. of high rainfall
D. with abundant
4.The Weddell Sea _____.
A.is a good example of increased salinity in freezing sea water
B.is much larger in area than the Arctic oceans
C.has a much lower salinity now than ever
D.has the denser water in its upper parts
5.Coastal regions are mentioned as cases where _____.
A.sea water is less salty because fresh water joins in
B.rivers carry industrial exhaust into sea
C.sea ice tends to melt more quickly than in the center of oceans
D. heavy water sinks to the deeper portions of the oceans
1.[B] 主旨大意題。本文第1段第3句是全文的主題句，告訴讀者有三種原因致使海洋鹽分變化并逐一介紹這三種原因，選項(xiàng)B比較全面地歸納了本文的主題。選項(xiàng)A和C中心詞為oceans,不妥，選項(xiàng)D只提及了前面兩種原因，不全面。
2.[A] 亊實(shí)細(xì)節(jié)題。只要明白第1段第5句中的in this manner是指第4句中的evaporation,不難做出正確選擇。選項(xiàng)B、C會(huì)使含鹽量減少,很容易排除。具體的反證分別出現(xiàn)在第2段首句、第3段尾句和第4段首句。
3.[C] 推理判斷題。綜合3種情況，造成海洋含鹽量低的原因有：降雨多、日曬少、遠(yuǎn)離熱帶和海冰融化，由此可推斷選項(xiàng)C最符合。A和D是使海水含鹽量增加的，因此可排除。B是否一定海水含鹽低，文中并未提到。
4.[A] 推理判斷題。選項(xiàng)A是對(duì)最后一段第1句的同義改寫(xiě)。同一句可知選項(xiàng)C錯(cuò)。選項(xiàng)B根本沒(méi)有提及。文章最后一句表明D正好與亊實(shí)相反。
5.[A] 推理判斷題。coastal regions字眼在文章中提到兩遍，分別在第2段第3句及第3段最后一句。只要能夠迅速査找到任何一處均可做出正確判斷為A。B和C無(wú)原文支持;D張冠李戴，原文提及D內(nèi)容涉及的是Weddell Sea,而非coastal regions。
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