What are the two main purposes of pressurizing aircraft cabins?

Pressurizing aircraft cabins serves two key purposes that are integral to safe and comfortable flight operations. First, it creates a suitable environment for passengers and crew at high altitudes where the atmospheric pressure is significantly lower than at sea level. At cruising altitudes, the air pressure outside the aircraft is much less than the pressure required for humans to breathe normally. By maintaining a defined cabin pressure, the aircraft ensures that adequate oxygen levels are available for passengers, preventing hypoxia, which can lead to severe health issues.

Second, pressurization allows aircraft to operate efficiently at high altitudes. Flying at higher altitudes reduces air resistance and allows for better fuel efficiency, as thinner air creates less drag. However, without pressurization, operating at these altitudes would be unsafe and uncomfortable for occupants due to reduced oxygen levels and potential physical harm from the lower external pressure.

The other options, while touching on various aspects of aviation, do not capture the primary reasons for cabin pressurization effectively. Therefore, the answer focuses on the critical need to maintain a safe and breathable environment for humans while enabling the operational advantages that come with high-altitude flight.