A．no one has ever clearly known how he himself was bit.
B．man tends to be subjective when he describes his encouhter with a snake.
C．the aggressiveness of the snakes diversify according to different situations.
D．a snake does not begin to attack without provocation.
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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.
A．The appearance of vertebrates.
B．The point when human beings consciously chose to do something.
C．The coming into being of the photosynthetic cells.
D．The origins of the universe,
A．it is one part of human history:
B．it should be included in the history of the universe.
C．it can partly explain the way of life of the people in the past.
D．it decided the human history,