Why are both spreading and sinking possible at the surface in Convectional Current Theory?

Correct answer: Because currents rise at some places and descend at others

Direct answer: A. In the convectional current theory, heat differences inside the Earth produce movement in mantle material. When heated material becomes less dense, it rises; when it cools and becomes denser, it sinks. Rising currents can push material apart and produce spreading, while descending currents can pull material downward and produce sinking or convergence. Thus both movements are possible because currents rise in some places and descend in others. Option A correctly describes this two-way circulation. Option B is wrong because equal rainfall is a weather process and does not create mantle convection. Option C is wrong because river flow is surface drainage, not the cause of tectonic convection. Option D is wrong because mountains do not always melt, and melting is not the explanation for the circulation. Memory cue: hot and light rises; cool and heavy sinks. The supplied answer is scientifically suitable for the school-level theory.