Any observant person has noticed that a wild animal will allow a man or other potential enemy to approach only up to a given distance before it flees. “Flight distance” is the term used for this interspecies spacing. As a general rule, there is a positive relationship between the size of an animal and its flight distance—the larger the animal, the greater the distance it must keep between itself and the enemy. An antelope will flee when the enemy is as much as five hundred yards away. The wall lizard’s(壁虎) flight distance, on the other hand, is about six feet. Flight is the basic means of survival for mobile creatures.
　 Critical distance apparently is present wherever and whenever there is a flight reaction. “Critical distance” includes the narrow zone separating flight distance from attack distance. A lion in a zoo will flee from an approaching man until it meets a barrier that it cannot overcome. If the man continues the approach, he soon penetrates(进入) the lion’s critical distance, at which point the cornered lion turns back and begins slowly to stalk (逼近) the man.
　 Social animals need to stay in touch with each other. Loss of contact with the group can be fatal(致命的) for a variety of reasons including exposure to enemies. Social distance is not simply the distance at which an animal will lose contact with his group—that is, the distance at which it can no longer see, hear, or smell the group—it is rather a psychological distance, one at which the animal apparently begins to feel anxious when it goes beyond its limits.
Social distance is not always rigidly(刻板的) fixed but is determined in part by the situation. When the young of apes and humans are mobile but not yet under control of the mother’s voice, social distance may be the length of her reach. When added control is needed because of danger, social distance shortens. To show this in man, one has only to watch a family with a number of small children holding hands as they cross a busy street.
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Which of the following is the correct explanation of “Flight Distance” in paragraph 1?

A．Distance between animals of the same species before fleeing.

B．Distance between large and small animals before fleeing.

C．Distance between an animal and its enemy before fleeing.

D．Distance between certain animal species before fleeing.

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If a lion’s critical distance is penetrated, it will __________.

A．begin to attack.

B．try to hide.

C．begin to jump.

D．run away.

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. The example of “children holding hands when crossing the street” in the last paragraph shows that ________.

A．social distance is not always needed.

B．there is no social distance among small children.

C．humans are different from animals in social distance.

D．social distance is sometimes determined by outside factors.

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. Which of the following can be the best title of the passage?

A．Critical Distance

B．Social Distance

C．Relationship Between Animals

D．Spacing in Animals