Explanation: Secondary cells, often called rechargeable batteries, are fundamentally defined by their ability to be recharged (A). This means the chemical reactions that occur during discharge can be reversed by applying an external current, restoring the cell to its charged state. For this reversibility to be efficient and effective, the products of the chemical change must remain largely insoluble in the electrolyte (C). If the reaction products were soluble and diffused away into the electrolyte, or if they formed irreversible compounds, the cell would not be able to effectively regenerate its original active materials on the electrodes during charging. This insolubility ensures the active materials stay localized and available for the reverse reaction. Therefore, both the ability to be recharged and the insolubility of the reaction products are key characteristics of secondary cells, making D the correct answer. B is incorrect because secondary cells *can* be recharged.