In an era of underdeveloped productivity, war has become the most important way to compete for resources. Youth are the most important participants in the war, and to win the war, young people must have strong physical fitness. In order to win the war, each City-state youth organized to train. As the leader of all Greece, the Athens city-state had a population of 250,000 during its heyday. At the same time, Corinth had a population of 90,000, and some city-states had only 5,000 or less. The larger the population, the higher the proportion of youth. The more the city-state is able to occupy an advantageous position in the competition. This passage emphasizes that, in an era of underdeveloped productivity:. ['Youth play an important role in the competition.', 'Access to resources mainly depends on the way of war.', 'Population size is an indicator of urban development.', "The competition mainly depends on people's physical fitness."]