They are farther apart shows the motion of a group of water molecules in diagram over a heat source. Thus the correct option is A.
What is motion?
Motion is defined as the change with time of the position or orientation of a body. Motion which changes the orientation of a body is called rotation.
Warmer liquid rise so which means that A is hotter and B is colder this is way B is moving down. This means that C and D can not be correct, and the molecules do not grow.
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