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  3. The Snake’s Forked Tongue 1. One of the most intriguing physical characteristics of snakes is their forked tongues. While thi

The Snake’s Forked Tongue 1. One of the most intriguing physical characteristics of snakes is their forked tongues. While thi

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问题                 The Snake’s Forked Tongue
   1. One of the most intriguing physical characteristics of snakes is their forked tongues. While this trait is also shared by some types of lizards, all snake species exhibit split tongues, which usually have a more pronounced fork than those of lizards. Any observer of snakes has no doubt noticed that they often stick their tongues out of their mouths, flick them up and down in the air, and then retract them. In fact, due to a slight indentation in the upper jaw, many species can protrude their tongues without even opening their mouths.
   2. The explanation for this behavior remained unknown for centuries, giving rise to many folk superstitions. For example, the forked tongue has become synonymous with deceit, from the idea that two separate tongues would enable a creature to voice two contrasting ideas at the same time. Another popular misconception is that snakes sting other animals with their tongues. Recently, though, scientists uncovered the truth about the forked tongue, and its actual use is much less sinister.
   3. Snakes differ greatly from humans on a sensory level. On the one hand, their eyesight is poor and they lack outer ears so they cannot hear with much volume or clarity, but, on the other, some have the ability to "see" and hunt in the dark by sensing the body heat of their prey. In addition, snakes have a method of smelling that is peculiar in that it is actually performed by the tongue rather than by the nostrils and nasal cavities, which in fact undertake other functions, such as breathing. When a snake flicks its tongue around in the air, it is actually snaring chemical particles, which can be thought of as odors. Next, the snake brings its tongue back in and places it against the roof of its mouth, where there are two small openings that correspond to the two tips of the forked tongue. These make up the snake’s Jacobson’s organ, a kind of chemoreceptor that analyzes the chemical odors on the tongue and sends this information to the snake’s brain.
   4. This technique for "smelling" the surrounding environment is truly unique, but even more amazing is the reason why the snake’s tongue is forked. By spreading the tips apart when the tongue is flicked out, each samples chemical particles from a slightly different position in the air and brings them back to the Jacobson’s organ.The organ then determines which tip was exposed to a larger amount of the particles in question, and this data tells the snake where the source of the odor can be found. In other words, if the tongue’s right tip contains more of the particles, such as those of an animal’s chemical odor, the snake knows that to find the animal it must travel to the right, not to the left.
   5. Obviously, the ability of snakes to pursue scents in this fashion is very useful in hunting,A It has been proven to be quite accurate, allowing them to pick up the trail of rodents and other prey even though the animal itself may be a great distance away.B Similarly, if the odor the snake perceives belongs to a predator, it is able to adjust its course to steer clear of the danger, C Another suggested use of snakes’ sense of smell is that it aids them in ascertaining the locations of communal dens*, where snakes reside in groups during the winter months.D
   6. However, scientists who have studied the chemoreception process in snakes believe its most vital role is in the detection of mates. Because most snakes lead solitary lives for much of the year, the capability of males to track down females during the mating season is essential for the survival of snake species. A male does this by sampling the air for the pheromones of a female. The chemical odor given off by the female gives the male all the information he needs, including whether she is of the same species, whether she is ready to mate, and in which direction she is traveling. Without the olfactory use of their tongues, it would be much harder for snakes to reproduce.
   den* the home of certain types of wild animals such as lions or foxes
The word pronounced in the passage is closest in meaning to
选项 A、controlled.
B、obvious.
C、evolved.
D、useful.
答案B
解析
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