[单项选择]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 into 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
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.