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Although high frequency signals may be received from a distant station by a sky wave at a certain time, it may not be possible to hear them an hour later. This may be due to:

  a.   changes in the ionosphere

  b.   shading of the earth by clouds

  c.   changes in atmospheric temperature

  d.   absorption of the ground wave signal
Your answer "b" is not correct. "a" is the right answer.

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