2. 外语
  3. Petroleum products,such as gasoline,kerosine,home heating oil,residual fuel oil,and lubricating oils, come from one source-crude

Petroleum products,such as gasoline,kerosine,home heating oil,residual fuel oil,and lubricating oils, come from one source-crude

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问题     Petroleum products,such as gasoline,kerosine,home heating oil,residual fuel oil,and lubricating oils, come from one source-crude oil found below the earth’s surface,as well as under large bodies of water, from a few hundred feet below the surface to as deep as 25,000 feet into the earth’s interior. Sometimes crude oil is secured by drilling a bole through the earth,but more dry boles are drilled than those producing oil. Pressure at the source or pumping forces crude oil to the surface.
    Crude oil wells flow at varying rates,from ten to thousands of barrels per hour. Petroleum products are always measured in 42-gallon barrels.
    Petroleum products vary greatly in physical appearance:thin,thick,transparent or opaque,but regardless, their chemical composition is made up of only two elements:carbon and hydrogen,which form compounds called hydrocarbons. Other chemical elements found in union with the hydrocarbons are few and are classified as impurities. Trace elements are also found,but these are of such minute quantities that they are disregarded. The combination of carbon and hydrogen forms many thousands of compounds which are possible because of the various positions and joinings of these two atoms in the hydrocarbon molecule.
    The various petroleum products are refined from the crude oil by heating and condensing the vapors. These products are the so-called light oils,such as gasoline,kerosene,and distillate oil. The residue remaining after the light oils are distilled is known as heavy or residual fuel oil and is used mostly for bunting under boilers. Additional complicated refining processes rearrange the chemical structure or the hydrocarbons to produce other products, some of which are used to upgrade and increase the octane(辛烷)rating of various types of gasoline.
Many thousands of hydrocarbon compounds are possible because_____.
选项 A、the petroleum products vary greatly in physical appearance
B、complicated refining processes rearrange the chemical structure
C、the two atoms in the molecule assume many positions
D、the pressure needed to force it to the surface causes molecular transformation
答案C
解析 文章第三段解释了碳和氢结合能形成数千种化合物的原因是“the various positions and joinings ofthese two atoms in the hydrocarbon molecule”，选项C符合文意。
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