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下面的短文后有2项测试任务：（1）第23～26题要求从所给的6个选项中为第l，2，3．5段每段选择1个最佳标题；（2）第27～30题要求从所给的6个选项中为每个句子确定1个最佳选项。
Spacing in Animals
1 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 terms used for this interspecies spa~ng. 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 flight distance， on the other hand， is about six feet. Flight is the basic means of survival for mobile creatures.
2 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 the lion"s critical distance， at which point the cornered lion reverses direction and begins slowly to stalk the man.
3 Social animals need to stay in touch with each other. Loss of contact with the group can be fatal for a variety of reasons including exposure to enemies. Social distance is not simply the dis-tance 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 he exceeds its limits. We can think of it as a hidden band that contains the group.
4 Social distance varies from species to species. It is quite short—apparently only a few yards——among some animals， and quite long among others.
5 Social distance is not always rigidly 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. This is readily observed among the baboons in a zoo. When the baby approaches a certain point， the mother reaches out to seize the end of its tail and pull it back to her. When added control is needed because of danger， social distance shrinks. 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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