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Discovered a mere one hundred fifty years ago and manufactured commercially just half that long, aluminum today ranks behind only iron and steel among metals serving mankind. The key to its popularity is its incredible versatility. The same metal that makes kitchen foil serves as armor for battlefield tanks. The material of lawn chairs and baseball bats also forms the vital parts of air and space vehicles -- most of their skeletons, their skins, even the rivets that bind them together.
Behind aluminum's versatility lie properties so diverse that they almost seem to belong to several different metals. For example, in its pure form, aluminum is soft enough to whittle. Yet its alloys can possess the strength of steel, with only a third of its weight. This, when Alexanedr Calder designed one of his last mobiles--a soaring creation eighty feet long -- his choice of aluminum over steel cut two tons from its weight. Aluminum also assures the masterpiece virtual immortality. The instant the metal is exposed to air, its surface acquires a transparent film that seals the interior against further corrosion.
69. According to the passage, aluminum is sometimes used to make which of the the following?
A. Photographic film.
B. Frames for stuffed furniture.
C. Foils and other fencing equipment.
D. Parts of spaceships.
正确答案是
70. The most important feature of aluminum is that it ______.
A. can be used for many different things
B. is soft enough to whittle
C. is strong
D. can be marketed in pure form
正确答案是
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