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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.

According to the passage, why was aluminum considered a precious metal between 1825 and 1886?

  • AAluminum ores were extremely costly to mine and ship.
  • BTerrestrial sources of aluminum oxide were as yet undiscovered.
  • CMetallic aluminum was known to be highly reactive.
  • DThe expense of isolating the pure metal was exorbitant.
  • EAluminum exhibited faster rates of oxide formation than many other metals.
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正确答案: D

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