A major reason for conflict in the animal world is territory. The male animal (21) an area. The size of the area is sufficient to provide food for him, his (22) and their offspring. Migrating birds, for example, (23) up the best territory in the order of "first come, first (24) ." The late arrivals may acquire (25) territories, but less food is available, or they are too close to the (26) of the enemies of the species. (27) there is really insufficient food or the danger is very great, the animal will not (28) . In this way, the members of the species which are less fit will not have offspring.When there is conflict (29) . territory, animals will commonly use force, or a (30) of force, to decide which will stay and which will go. It is interesting to note, however, that animals seem to use (31) the minimum amount of force (32) to drive away the intruder. There is usually no killing. In the (33) of those animals which are capable of doing each other great harm, (34) is a system for the losing animal to show the winning animals that he (35) to submit. When he shows this, the (36) normally stops fighting. Animals (especially birds), which can easily escape from conflict seem to have (37) obstacle against killing, and equally no mechanism (38) submission. The losing bird simply flies away. However, if two doves are (39) in a cage, and they start fighting, they will continue to fight until one kills the other. We all think of the dove as a symbol of peace and, in its natural habitat, it is peaceful. But the "peace" mechanism does not (40) in a cage. 21（）

A. establishes
B. builds
C. founds
D. erects