Concept:A unique natural advantage of California helped its aircraft industry grow.
Explanation:California, especially its southern region, enjoys clear skies and mild weather almost all year.
This steady clear weather makes it perfect for safely testing and flying aircraft.
Such a natural condition is special to California.
Cheap labour, a peaceful atmosphere, and government support are not exclusive to California.
They can be found in many other places too.
Therefore, the clear sky is the distinctive factor for the aircraft industry there.
Answer:D. Clear sky which favours testing of aircrafts