Final answer:
Auxiliary Power Units (APUs) are environmentally friendly and cost friendly because they use less fuel than an idling engine, leading to better fuel efficiency and reduced emissions. Technologies like APUs, when used appropriately, complement other advances aimed at improving fuel economy and reducing environmental impact.
Step-by-step explanation:
Auxiliary Power Units (APUs) are considered to be environmentally friendly and cost friendly primarily because they use less fuel than an idling engine. This means they generate fewer emissions when compared to an engine that is left running idle. APUs are used in a variety of settings, including in aircraft, trucks, and in heavy equipment, to provide energy for functions such as heating, cooling, and electrical power when the main engine is not running.
APUs contribute to better fuel efficiency and reduced emissions by providing necessary power without the need to keep the main engine running. This is in line with advances in engine technologies, such as computer controls, variable valve timing, and gasoline direct injection, which aim to improve fuel economy and reduce CO2 emissions. Furthermore, when considering the reduction of air pollutants, it's important also to factor in that catalytic converters, alongside unleaded gasoline and low sulfur levels, significantly reduce hydrocarbon and nitrogen oxide emissions, which are harmful to the environment.
Therefore, option B is correct, while options A and C are not applicable to the environmentally and cost-friendly nature of APUs, making them less relevant answers to this question.