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Naturally，in a group of animals as diverse as the snakes, and with so many varied enemies, there are numerous defensive reactions and devices. There is, however, one general pattern of behavior. In the presence of suspected enemy the first reaction is to try to escape observation; if this fails, the next resort is the flight to some inaccessible retreat, but if this is not possible, or is circumvented, various kinds of intimidatory gestures and warning devices are brought mto play; in the last resort the snake attacks. This pattern varies with the circumstances ; some stages may be omitted or combined unpredictably whilst ; some notoriously irascible species may dispense with all the preliminaries and attack almost at once, though seldom or never without some provocation.
Amongst the factors that increase aggressiveness are hunger, the mating season and surprise, with the last mentioned the commonest; when hunting for food or for mate, activity and the aggressive instinct are both at their peak. Owing to their poor sense of hearing snakes are very liable to be, quite literally, caught napping and a similar situation arises during their periods of temporary blindness just before sloughing (虫兑皮)begins. By far the greatest number of snake-bit accidents result from the unwitting disturbance of resting snakes, and this hazard is much increased with species that are well disguised and whose natural instinct is to trust to this concealment as their principal defense. As well as differences in aggressiveness between individuals of the same species according to the circumstances and conditions, there are also notable differences between species, even closely allied species ; and the reports of those who have been attacked may understandably be lacking in objectivity. So it is impossible to forecast, even in outline, how any encounter will develop.
The Hamadryad, for example, is usually credited with being amongst the most aggressive of snakes, and there are many accounts of unprovoked attacks, yet on one occasion fourteen men and seven dogs passed and returned within two yards from a nest and no snake was seen although the female, which guards the nest, could not have been far away.
21. When a snake meets a potential enemy, its primary device is .
A. avoiding any observation B. finding a safe shelter
C. giving a warning threat D. starting a quick attack
22. By “dispense with all the preliminaries”(Line 7，Para. 1) , the author most probably means that .
A. the snakes combine all the previous three steps
B. the snakes give up all the previous three steps
C. the snakes follow all the previous steps one by one
D. the snakes reverse the order of all the previous steps
23. A snake is most aggressive when .
A. it meets a possible enemy B. it is caught when it is sleeping or sloughing
C. it is disturbed unknowingly D. it seeks a partner in the mating season
24. It is difficult to predict what would happen when a snake meets a man because .
A. no one has ever clearly known how he himself was bit
B. man tends to be subjective when he describes his encounter with a snake
C. the aggressiveness of the snakes diversifies according to different situations
D. a snake does not begin to attack without provocation
25. The author uses the example in the last paragraph to support his idea that .
A. snakes do not begin the aggressive act if it is not disturbed
B. some snakes are more aggressive and more ready to attack
C. it is hard to forecast whether and how snakes would attack
D. snakes often conceal themselves as their principal defense