Answer:
From A to B, the addition of heat adds kinetic energy to the particles, which makes them vibrate faster (in their fixed positions in a lattice since its a solid). The temperature increases steadily since it is a measure of the average KE of the particles. From B to C, energy is used to overcome intermolecular forces of attraction, not to increase energy, so the temperature stays constant. The substance becomes a liquid at this stage, so the particles are no longer constrained in their fixed lattice and can slide over each other. From C to D it is similar to that of A to B. The avg KE of the particles increase and they move faster.
