Question

The sky appears dark to passengers flying at very high altitude

Hint:

The sky appears blue due to scattering of sunlight by air molecules. At high altitudes, there is less scattering, so the sky appears dark.

Answer 1

At high altitudes, the atmosphere becomes thinner, and there are fewer air molecules to scatter sunlight.
Scattering of light is what causes the blue color of the sky at ground level, but at higher altitudes, this scattering is much weaker, and thus, the sky appears dark. Thus, the sky appears dark to passengers flying at very high altitude because there is less scattering of light.