Where would someone be likely to see a moving airplane?
Options:
- military base
- airport
- sky
- airplane hangar
- hanger
Let's think carefully first. Step-by-step reasoning process:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Sky is the empty area above the land. Airplanes are vehicles used to fly above land. Moving airplane means flying above land. So the final answer is sky.