Why are dark clouds often connected with rainfall?

Correct answer: They may be dense and full of water

Correct answer: A, dark clouds may be dense and rich in water. A cloud looks white or grey depending on how light passes through and scatters among its droplets and ice crystals. A thick cloud contains a greater depth of water droplets and ice particles, so less sunlight passes through it. From below, it may appear dark or nearly black. Such a dense cloud can contain enough moisture for droplets or ice particles to grow and fall as rain or other precipitation. However, dark appearance alone does not guarantee rainfall; it is a useful visual clue, not an absolute rule. B is wrong because dark clouds are not always empty. C is wrong because ordinary rain clouds are not made only of dust; their important visible particles are water droplets and/or ice crystals. D is wrong because clouds do not end the air. A is best because it connects darkness with thickness, reduced light transmission, and possible water content. Memory cue: “thicker cloud blocks more light and may carry more rain.”