For a crystal fractionally melting in a Solid Solution Phase Diagram, beginning with a 50% rock.
Accept or Reject: the longer we let the melting go on before separating the fractions, the more Na rich the one fraction becomes and the more Ca rich the other fraction becomes.
Answer: Reject. Because the system remains in equilibrium until fractionation actually occurs, the melt will become more and more Ca rich the more the crystal melts. This is because for more crystal to melt the temperature must get higher, and the higher the temperature gets the more Ca goes into solutiuon and, thus, the more Ca rich the melt becomes.
Or, think about it this way, during both crystallization and melting an equilibrium system moves in tandem down (or up) the solidus and liquidus lines together. That means their compositions are shifting together in the same direction. The Accept/Reject statement is saying the compositions will move apart.