Which of the following statements concerning nickel-cadmium batteries is true? A. When mixing electrolyte always add acid to the water. B. Nickel-cadmium batteries can be charged and discharged only a limited number of times without damage to the cells. C. Nickel-cadmium batteries can be stored for a long time and still keep a full charge without additional charging. D. The electrolyte of an idle nickel-cadmium battery must be replaced monthly to maintain battery condition.