D.
variable-ratio schedule of reinforcement.
At room temperature or about 25°C, potassium nitrate (KNO₃) has a solubility of about 38.7 grams per 100 mL of water. At this concentration, the KNO₃ solution is said to be saturated since it already contains the maximum amount of solute that the solvent can dissolve, thus, if you add more KNO₃ salt, it will no longer dissolve. When the solution has less solute than that of the saturation concentration, it is called an unsaturated solution, thus if you add more solute, the KNO₃ salt will still dissolve. If the concentration of the solution is greater than its solubility, the solution is said to be supersaturated, thus, if excess KNO₃ salt is added, crystals will start to form.
