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  3. How it came about that snakes manufactured poison is a mystery. Over me periods their saliva, a mild, digestive juice like our o

How it came about that snakes manufactured poison is a mystery. Over me periods their saliva, a mild, digestive juice like our o

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问题     How it came about that snakes manufactured poison is a mystery. Over me periods their saliva, a mild, digestive juice like our own, was converted into a poison that defies analysis even today. It was not forced upon them by the survival competition: they could have caught and lived on prey without using poison just as the thousands of non-poisonous snakes still do. Poison to a snake is merely a luxury: it enables it to get its food with very little effort, no more effort than one bite. And why only snakes? Cats, for instance, would be greatly helped: no running fights with large, fierce rats or struggles with grown rabbits—just a bite and no more effort needed. In fact, it would be an assistance to all the carnivores—though it would be a two-edged weapon when they fought each other. But, of the vertebrates, unpredictable Nature selected only snakes(and one lizard). One wonders also why Nature, with some snakes concocted poison of such extreme power.
    In the conversion of saliva into poison one might suppose that a fixed process took place. It did not: some snakes manufactured a poison different in every respect from that of others, as different as arsenic is from strychnine, and having different effects. One poison acts on the nerves, the other on the blood.
    The makers of the nerve poison include the Mambas and the Cobras and their venom(毒液)is called neurotoxic(毒害神经毒液). Vipers(Adders)and Rattlesnakes manufacture the blood poison, which is known as haemolytic(溶血毒液). Both poisons are unpleasant, but by far the more unpleasant is the blood poison. It is said that the nerve poison is the more primitive of the two, mat the blood poison is, so to speak, a newer product from an improved formula. Be that as it may, the nerve poison does its business with man far more quickly than the blood poison. This, however, means nothing. Snakes do not acquire their poison for use against man but for use against prey such as rats and mice, and the effect on these of viperine poison is almost immediate.
Snakes use their poison against______.
选项 A、bigger animals
B、human beings
C、their environment
D、animals that are their food
答案D
解析 由第一段可知，蛇用毒袭击作为它们食物的动物。
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