G3C04

What does the term “critical angle” mean, as applied to radio wave propagation?

1. AThe long path azimuth of a distant station
2. BThe short path azimuth of a distant station
3. CThe lowest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
4. DThe highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions

Pick an answer to check it.