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E3B10

What is the cause of gray-line propagation?

Select	Answer
A	At mid afternoon the sun heats the ionosphere, increasing radio wave refraction and the MUF
B	At twilight solar absorption drops greatly while atmospheric ionization is not weakened enough to reduce the MUF
C	At darkness solar absorption drops greatly while atmospheric ionization remains steady
D	At midday the sun, being directly overhead, superheats the ionosphere causing increased refraction of radio waves

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