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  3. Nature has devised many ways to protect creatures’ eyes. The most common protection is the eyelid--a fold of skin that closes ov

Nature has devised many ways to protect creatures’ eyes. The most common protection is the eyelid--a fold of skin that closes ov

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问题     Nature has devised many ways to protect creatures’ eyes. The most common protection is the eyelid--a fold of skin that closes over the eye, protecting it from damage. Eyelashes are useful for keeping out dust and other irritants, and tears wash away any particles that get through the other defenses.
    Some creatures, including most birds, have three eyelids. The upper and lower lids act like human lids and keep out twigs, dirt and sand. The third eyelid, however, is a semitransparent tissue that crosses over the eye from the inside corner to the outside corner. Because of this protective membrane, birds seldom have to blink. They close their eyes only when they go to sleep. In ducks, this third eyelid self as an underwater diving mask that helps the ducks find food.
    Most fish and snakes have no eyelids at all. Instead, a hard glassy covering protects their eyes. In fish, water constantly sweeps away dirt from the covering. And a snake’s eyesight is usually so bad that a little dirt obscuring its vision does not disturb it greatly.
    Eyelashes defend the eye by shading it from glare. They also act like miniature brushes to remove dust. Camels have lashes that are four inches long to protect their eyes from windblown sand in the desert.  
The author compares the duck’s third eyelid to a ______.
选项 A、tissue
B、glass covering
C、tiny brush
D、diving mask
答案D
解析 第二段最后。鸭子的第三层眼皮如同潜水面罩。选项D是正确的。
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