Which of the following best explains the motion represented by the
arrows?
a. Rock is heated near the core at Point A, and therefore becomes less dense. It rises
towards Point B. As it moves from Point B to Point C, it is cooled and becomes denser,
so it sinks at Point D.
b. Rock is cooled near the core at Point A, and therefore becomes more dense. It rises
towards Point B. As it moves from Point B to Point C, it is heated and becomes less
dense, so it sinks at Point D.
c. Rock is heated near the core at Point A, and therefore becomes more dense. It rises
towards Point B. As it moves from Point B to Point C, it is cooled and becomes less
dense, so it sinks at Point D.
d. Rock is heated near the core at Point A, and therefore becomes less dense. It rises
towards Point B. As it moves from Point B to Point C, it is cooled and becomes less
dense, so it sinks at Point D.

A. Rock is heated near the core at Point A, and therefore becomes less dense. It rises towards Point B. As it moves from Point B to Point C, it is cooled and becomes denser, so it sinks at Point D.

Explanation:

The process illustrated is a convection current, something seen places like the earth's mantle and atmosphere. The hotter matter gets, the more excited its molecules become and the more they become expanded and distant from each other. (Bear in mind that there is the same amount of matter but it takes up more space, i.e. less dense). This is because the molecules are excited by the thermal energy and bounce off each other, pushing them apart.

Cold, closer to the surface (or outer space in terms of air), in turn causes the matter to shrink back into itself, which leads to the increase in density that causes it to sink and repeats the process.