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Which one of the following statements about cultural code is TRUE？
A major reason for 【21】______ 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 【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 larger territories, but less food is 【25】______ , 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 mate. In this way, the members of the species which are less fit will not have 【28】______ .
When there is conflict 【29】______ territory, animals will commonly use force, or 【30】______ of force, to decide who will stay and who will go. It is interesting to note, however, that animals seem to use the 【31】______ amount of force necessary to drive away the 【32】______ There is usually no killing. In the case of those animals which are 【33】______ of doing each other great harm, (here is a system for the 【34】______ animal to show the winning animal that he wishes 【35】______ When he shows this, the 【36】______ normally stops fighting. Animals especially birds, which can easily escape from conflict seem to have no obstacle against killing, equally no mechanism 【37】______ sub mission. The defeated bird simply flies away. However, if two doves are 【38】______ 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 【39】______ of peace and, in its natural habitat, it is peaceful. But the" peace" mechanism does not 【40】______ in a cage.
【21】
Why does the author say reducing the amount of meat in one's diet is environmentally beneficial？
听力原文： Although I think the United States generally has an excellent system of transportation, I do not think that it does a good job of transporting people between cities that are only a few hundred miles apart. A person commuting between Detroit and Chicago, or between San Francisco and Los Angeles may spend only a relatively short time in the air, while spending several hours getting to and from the airport. This situation makes flying almost as time-consuming as driving. Moreover, airplanes use a lot of their fuel just getting into the air. They simply are not fu- el effective on short trips.
High speed trains may be an answer. One fairly new proposal for such trains is for some- thing called a Maglev, meaning a magnetically levitated train. Maglev will not actually ride on the tracks, but will fly above tracks that are magnetically activated. This will save wear and tear on the tracks. These trains will be able to go faster than 150 miles per hour. At this speed, conventional trains have trouble staying on the tracks.
As you can see, Maglev offers exciting possibilities for the future.
What is the main topic of the talk？
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