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  3. A metal with a built in memory is one of the very useful discoveries made during America’s space program. Nitinol was discovered

A metal with a built in memory is one of the very useful discoveries made during America’s space program. Nitinol was discovered

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问题     A metal with a built in memory is one of the very useful discoveries made during America’s space program. Nitinol was discovered by scientists working at the Naval Surface Weapons Center. They were looking for heat shield metals for missiles and satellites, but what they found was nitinol. This alloy can be formed into shapes just like any other metal, but it returns to its original form at a certain temperature.
    When this alloy of nickel and titanium is first formed, it is shaped into ingots, brick shaped masses of the metal. Then it is drawn out into a wire of varying thickness, depending on the job it is destined to do. It is set into its desired curve or shape and heated briefly. It is then cooled off by dipping the wire in cold water. This "sets" its memory. The metal can then be formed into any desired shape, but whenever its temperature exceeds its room temperature, ’ it returns to the shape it has memorized.
    This makes nitinol perfect, of course, for, spacecraft, for antennas, probes, shields, and folding "arms" made of nitinol can be stored safely inside a spacecraft, protecting them from damage during launching. Then, once safe in orbit, the spacecraft would suddenly "grow" antennas and other structures. The sun’s rays would have warmed up the folded and coiled nitinol, and it would have returned to its original shape.
    But scientists have found a more down to earth use for this wire: they now use it to strengthen teeth. Before nitinol, teeth that needed straightening have brackets attached to them; wires were attached to these to exert a pull in the right direction. But this pull must be constant, and the stainless steel arch wire cannot exert the correct force if it has been bent several times or if the tooth has actually moved toward it as planned. As a result, many people with dental braces must have the wire replaced monthly, tightened every few weeks, or adjusted painfully tight at the beginning of the treatment.
    But nitinol wires can be formed to exert force in a certain direction, cooled of and then reformed to fit the patient’s mouth. As the mouth warms the nitinol, it begins to try to assume its preset shape. This exerts a steadier, more constant pull on the tooth than could ever be achieved with stainless steel wire. Nitinol can be bent into such intricate shapes — while maintaining its pull — that very difficult treatments, such as moving one tooth from behind another, can be done now without the expense or pain of constant adjustment. Nitinol has cut tile time needed for these treatments by almost half, a real benefit for both patient and dentist.
We can conclude from this passage that ______.
选项 A、nitinol has its "memory" function because it is an alloy
B、the sun’s rays are necessary to trigger the metal’s "memory"
C、people have only begun to discover the many uses of nitinol
D、one problem with nitinol is that it often returns to its original shape at inopportune time
答案C
解析 综合分析题。 A项：镍钛诺具有恢复原状的特性是因为它是合金；固然，镍钛诺合金具有恢复原状的特性，然而，是合金并不是它具有这种特性的直接原因，所以A项不正确。B项：要想引发这种金属的“记忆功能”，必须要有光照；文章最后1段第 2句指出，口腔温度可以温暖镍钛诺，使之呈现预先设定的形状，所以B项不正确。D项：使用镍钍诺，存在的问题是，它经常在不适宜的时候恢复原状，文中没有提及，所以不正确。C项：人们才刚开始发现镍钍诺的多种用途；由第3段第1句得知，镍钛诺可广泛用于宇宙飞船、天线、探测器、防护材料等，现在科学家们又发现镍钍诺可用于稳固牙齿，这说明人们对该种合金还并不是很了解，其用途正在不断发现和推广中，故C项正确。
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