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　　阅读判断(第16～22题，每题1分，共7分)

　　阅读下面这篇短文，短文后列出了7个句子，请根据短文的内容对每个句子做出判断。如果该句提供的是正确信息，请在答题卡上把a涂黑;如果该句提供的是错误信息，请在答题卡上把b涂黑;如果该句的信息文章中没有提及，请在答题卡上把c涂黑。

　　where has the salt come from?

　　every now and then, we meet a fact about our earth that makes us feel strange and no answer for the fact has yet been found. such a fact is the existence of salt in the oceans. how did it get there?

　　we simply do not know how the salt got into the ocean! we do know, of course, that salt is water-soluble, and so passes into the oceans with rainwater. the salt of the earth's surface is constantly being dissolved (溶解) and is passing into the ocean.

　　but we do not know whether this can explain the huge quantity of salt in oceans, if all the oceans were dried up, enough salt would be left to build a wall 180 miles high and a mile thick. such a wall would reach once around the world at the equator (赤道)!

　　the common salt that we all use is produced from seawater or the water of salt lakes, from salt springs (源泉) and from deposits of rock salt. the concentration (浓度) of salt in seawater ranges from about three per cent to three-and-one-half percent. the dead sea, which covers an area of about 340 square miles, contains about 11,600,000,000 tons of salt!

　　on the average, a gallon (加仑) of seawater contains about a quarter of a pound of salt. the beds of rock salt that are found in various parts of the world were all originally formed by the evaporation (蒸发) of seawater millions of years ago. it is believed that the thick rock-salt deposits were formed after about nine-tenth of the volume of seawater had been evaporated.

　　most commercial salt is obtained from rock salt. the usual method is to drill wells (井) down to the salt beds. pure "water is pumped down (抽进去) through a pipe. the water dissolves the salt and it is forced through another pipe up to the surface.

　　16 we have not fully understood how salt got into the ocean.

　　a　right b　wrong c　not mentioned

　　17 the author is sure that the dissolved salt from the earth's surface is the only source of the huge quantity of salt found in oceans.

　　a　right b　wrong c　not mentioned

　　18 if all the oceans were dried up, the salt thus obtained would be extremely great in size.

　　a　right b　wrong c　not mentioned

　　19 the percentage of salt content in the red sea is higher than that in the dead sea.

　　a　right b　wrong c　not mentioned

　　20 beds of rock salt are found in every part of the world.

　　a　right b　wrong c　not mentioned

　　21 after evaporation, about ten percent of seawater becomes rock salt.

　　a　right b　wrong c　not mentioned

　　22 most commercial salt is obtained from seawater.

　　a　right b　wrong c　not mentioned

　　参考答案：16.a　17.b　18.b　19.c　20.a　21.a　22.b