单选题
A major reason for conflict in the animal world is territory. The male animal establishes an area. The size of the area is sufficient to provide food for him, his mate and their offspring. Migrating birds, for example, used up the best territory in the order of"first come, first【C1】______". The late arrivals may acquire【C2】______territories, but less food is available, or they are too close to the【C3】______of the enemies of the species.【C4】______there is really insufficient food or the danger is very great, the animal will not【C5】______. In this way, the members of the species which are less fit will not have offspring. When there is conflict【C6】______territory, animals will commonly use force, or a show of force, to decide which will stay and which will go. It is interesting to note, 【C7】______, that animals seem to use only the minimum amount of force【C8】______to drive away the intruder. There is usually no killing. In the case of those animals which are capable of【C9】______each other great harm, 【C10】______is a system for the losing animal to show the winning animal that he wishes to submit When he shows this, the victor normally stops fighting.