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E3B10

What is the cause of gray-line propagation?

Select	Answer
A	At midday, the Sun super heats the ionosphere causing increased refraction of radio waves
B	In darkness, solar absorption drops greatly while atmospheric ionization remains steady
C	At mid-afternoon, the Sun heats the ionosphere decreasing radio wave refraction and the MUF
D	At twilight and sunrise, D-layer absorption is low while E-layer and F-layer propagation remains high

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