Explanation: The charge of a lead-acid storage cell can indeed be determined by both methods listed. A) **Hydrometer readings** measure the specific gravity (density) of the electrolyte, which is a mixture of sulfuric acid and water. When a lead-acid battery discharges, sulfuric acid in the electrolyte combines with the lead plates. This process makes the electrolyte less dense. Conversely, during charging, sulfuric acid is released back into the solution, increasing its density. Therefore, a higher specific gravity indicates a fuller charge. B) While the open-circuit voltage of a lead-acid battery remains relatively stable for much of its discharge cycle, measuring the **voltage under a heavy load** provides a much more accurate indication of its state of charge. A fully charged battery will maintain a higher voltage under load, whereas a discharged battery's voltage will drop much more significantly due to its increased internal resistance. Both A and B are reliable methods for assessing the charge of a lead-acid cell, making C the correct answer.