It is known that the substance has different 3 phases (solid, liquid, and gas).
By heating, the substance converted from solid phase to liquid phase passing by a plateau that represents equilibrium between the two phases.
By further heating, the solid completely converted into liquid phase, then converted to gaseous phase passing by a plateau that represents an equilibrium between the liquid an gaseous phase.
<em>The plateau at the lower temperature represent: B) Melting of a solid.</em>
