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  3. Petroleum products, such as gasoline, kerosine, residual fuel oil, and lubricating oils, come from a few hundred feet below the

Petroleum products, such as gasoline, kerosine, residual fuel oil, and lubricating oils, come from a few hundred feet below the

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问题     Petroleum products, such as gasoline, kerosine, residual fuel oil, and lubricating oils, come 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 hole through the earth, but more dry holes 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 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, kerosine, 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 burning under boilers. Additional complicated refining processes rearrange the chemical structure of the hydrocarbons to produce other products. Some of which are used to upgrade and increase the 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 第三段最后一句说“碳氢结合才有可能形成成千上万种化合物，因为这两种原子在碳氢分子中处于不同位置并且结合起来”。故与C相符。
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