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  3. Bird wings have a much more complex job to do than the wings of an airplane, for in addition to supporting the bird they must ac

Bird wings have a much more complex job to do than the wings of an airplane, for in addition to supporting the bird they must ac

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问题     Bird wings have a much more complex job to do than the wings of an airplane, for in addition to supporting the bird they must act as its engine, rowing it through the air. Even so the wing outline or a bird conforms to the same aerodynamic principles as those eventually discovered by people when designing airplanes, and if you know how different kinds of aircraft perform, you can predict the flight capabilities of similarly shaped birds.
    Short, stubby wings enable a tanager and other forest-living to swerve and dodge at speed through the undergrowth, just as they helped the fighter planes of the Second World War to make tight turns and acrobatic maneuvers in a dog-fight. More modern fighters achieve greater speeds by sweeping back their wings while in flight, just as peregrines do when they go into a 130kph dive, swooping to a kill. Championship gliders have long, thin wings so that, having gained height in a thermal up-current they can soar gently down for hours and an albatross, the largest of flying birds, with a similar wing shape and a span of 3 meters, can patrol the ocean for hours in the same way without a single wing beat. Vultures and hawks circle at very slow speeds supported by a thermal and they have the broad rectangular wings that very slow-flying aircraft have. People have not been able to adapt wings to provide hovering flight. That has only been achieved with the whirling, horizontal blades of a helicopter or the downward-pointing engines of a vertical landing jet. Hummingbirds have paralleled even this. They tilt their bodies so that they are almost upright and then beat their wings as fast as 80 times a second producing a similar downdraft of air. So the hummingbird can hover and even fly backwards.
Why can the hummingbird hover and even fly backwards?
选项 A、Because its wings are large enough.
B、Because it can tilt its body.
C、Because it has whirling and horizontal blades like a helicopter.
D、Because it can produce down-draft of air by beating their wings very fast.
答案D
解析 题目问：为什么蜂鸟可以盘旋飞行甚至可以倒退?“Hummingbirds have paralleled even this．Theytilt their bodies so that they are almost upright and then beat their wings as fast as 80 times a secondproducing a similar downdraft of air．So the hummingbird can hover and even fly backwards．”通过这句话可知，蜂鸟震动翅膀的次数可以达到80次／秒，这样就可以产生一股类似下降的气流，所以，蜂鸟可以盘旋飞行甚至可以倒退。据此判断，应选择D。
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