Why do clouds turn black when they're about to give us some rain?

Question

Why do clouds turn black when they're about to give us some rain?

Answer 1

They are full of rain and the sun's light cannot penetrate the clouds.

Answer 2

Because of the density of water in them.

Answer 3

because so much water droplest form around dust particules and then it is so much that it just holds for a day or to and when the time comes it rains.  

The clouds are Black because when you get a gray shirt or towel wet, what color does it turn.

Its as easy as that.

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