Final answer:
Towns and cities struggle to exist above 5 km in elevation due to low air pressure and the resulting lack of oxygen, as well as colder temperatures. These conditions make it challenging for sustainability and survival.
Step-by-step explanation:
It is difficult if not impossible for towns and cities to exist much higher than 5 km in elevation primarily because of low air pressure, which leads to not enough oxygen for survival (choice c). At higher altitudes, the atmosphere is thinner and the amount of oxygen in the air is considerably less than at sea level. This can lead to hypoxia, where body tissues do not receive enough oxygen, and can result in severe health problems or even death. Additionally, temperatures tend to be much colder at higher elevations, which can make it challenging for human habitation and agriculture.
Human adaptation requires a certain range of temperature and pressure, conditions that are not met at extreme altitudes. While climate and the presence of ozone in the stratosphere make Earth habitable by protecting us from ultraviolet radiation and maintaining tolerable temperatures, these conditions change with altitude, and life at high elevations would require significant adaptation that goes beyond our current capabilities.