The intersection in the (1) curve indicates that the solubility of the two substances is equal at this temperature, so the point m indicates that the solubility of the two substances A and B is the same at 50℃;

(2) A nearly saturated solution can be saturated by adding solute and evaporating solvent;

(3) When the saturated solution of A and B at 50℃ is cooled to 20℃ respectively, the solubility decreases and crystals will precipitate. The mass fraction of solute is related to the solubility at this temperature. As can be seen from the figure, the solubility of B is greater than that of A, so the solute mass fraction of B is greater than that of A. Because the solubility of C becomes larger after cooling and becomes an unsaturated solution, the solute mass fraction remains unchanged, and its solute mass fraction depends on the solubility at 50℃, because

(4) The solubility of the upper curve is relatively high. As can be seen from the figure, the temperature when B > A ≥ C is 20℃ ≤ T < 50℃.

So the answer is:

① The solubility of A and B is the same at 50℃;

(2) increase solute (or evaporate water at constant temperature);

③C；

④20℃≤t