单选题
There is probably no sound in (1) more chilling than the " hiss " of a diamondback rattlesnake. There is good reason: the western diamondback has killed more humans than (2) snake. It is easily excitable, very aggressive, frequently hungry, and deadly poisonous. Yet it doesn't really hiss. Rather, it (3) its tail. A mature rattler can move its tail up and down between fifty and a hundred times a second! The hollow rattles (4) the tail, beating against each other, create the "hissing" sound. Why does this creature (5) rattles? Rattlesnakes molt three times a year, shedding their outer layer of skin each time. But the molting skin near the tail is not discarded. (6) it hardens and hollows out, becoming another rattle. If a snake had never lost any rattles, you could (7) its age by counting them and dividing by three. Do the snakes need their rattles? The rattles cannot be used in a mating call, (8) rattlesnakes are deaf. They are not a sign of hunger, for snakes with full stomachs rattle as often as hungry (9) . And in the wild, the rattling scares prey. It does not (10) them. Therefore, scientists believe that snakes use rattles merely to warn larger animals not to step on (11) . They have studied them extensively and found that it was a function more important in ages past when the rattlers shared the plains (12) 60 million buffalo! Rattlesnakes are one of the (13) advanced forms of " pit " vipers—animals who possess an organ for an extra sense. The pit organ is like an infrared radar sensor. (14) in the snake's head, the pit organ can sense differences in temperature between inside and outside itself—differences as small as 1% of a degree. Not only can rattlesnakes (15) sense the presence of another animal—or a human—but they apparently can (16) determine its direction and range. These animals don't feed on (17) , of course. Their poison, however, may kill humans. But this happens only in (18) they think is self-defense. Rattlesnakes are really quite (19) Their principal diet of mice and rats makes them valuable to the ecology of the West. So rather than fear them, we should respect the (20) they play in containing the population of these harmful pests.