E3B10 (B)
What is the cause of gray-line propagation?
A. At midday, the Sun being directly overhead superheats the ionosphere causing increased refraction of radio waves
B. At twilight, D-layer absorption drops while E-layer and F-layer propagation remain strong 
C. In darkness, solar absorption drops greatly while atmospheric ionization remains steady
D. At mid afternoon, the Sun heats the ionosphere decreasing radio wave refraction and the MUF
