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The most common metal in the Earth`s crust, aluminum (or aluminium) was not discovered until 1825 because its isolated state is so reactive that free nuggets or flakes of the metal are never found in nature; rather, the metal is typically found as part of an amalgam, most commonly bauxite ore. Moreover, elemental aluminum is extremely difficult-and expensive-to separate from its ores by traditional chemical means. Indeed, the extreme reactivity of aluminum helps protect its modern, ubiquitous manifestations, such as aluminum foil. The surface of pure aluminum instantly combines with atmospheric oxygen to form a thin but robust "passivization" seal of aluminum oxide that prevents further corrosion. Many other metals, such as iron, are less reactive than aluminum, but their superficial oxides do not form as swiftly, completely, or impermeably.

For several decades after its discovery, aluminum was considered a precious metal and was more costly than gold or platinum, not because of any fundamental scarcity, but because of its elevated cost of production. The price of aluminum suddenly plummeted in 1886, however, when two 23-year-old inventors independently developed an electrolytic process of separating pure aluminum from a bath of molten aluminum salts, primarily cryolite. Cryolite itself is rare enough that synthetic salts eventually replaced it as the solution medium.

From the passage, it can be inferred that

  • Ametals found as free nuggets or flakes form passivization seals that inhibit corrosion
  • Bthe electrolytic process of producing aluminum is less expensive than traditional chemical processes
  • Cat least some precious metals are found in nature as free nuggets or flakes
  • Diron is rarely used as foil because its oxides do not form a rapid, robust seal
  • Emost metals that are commonly thought of as precious are scarce in the Earth`s crust
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正确答案: B

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