What altitude and temperature combination is expected to decrease lift efficiency in helicopters?

The choice reflecting high altitude and high temperatures as a combination that decreases lift efficiency in helicopters is accurate due to the fundamental principles of aerodynamics and how atmospheric conditions affect helicopter performance.

At high altitudes, the air density decreases, which leads to less lift being generated by the rotor blades. Helicopters rely on the density of air to create lift; thinner air provides less lift for the same rotor speed compared to denser air at lower altitudes. Additionally, when temperatures are high, the air expands, further reducing its density. The combination of these two factors exacerbates the challenges faced by helicopters, as the rotor must work harder to generate the same amount of lift, leading to potential performance limitations.

This understanding is essential for pilots and aviation professionals when planning flights, particularly in mountainous regions or during hot weather, as it directly impacts safety and operational effectiveness.