The correct answer is: C. both (A) and (B).
A constant-current system of charging a lead acid cell will charge the cell at a constant current until the cell reaches a certain voltage. At this point, the current will decrease and the voltage will remain constant until the cell is fully charged. A constant-voltage system of charging a lead acid cell will charge the cell at a constant voltage until the cell reaches a certain current. At this point, the voltage will remain constant and the current will decrease until the cell is fully charged.
The advantage of a constant-voltage system over a constant-current system is that it will reduce the time of charging. This is because the cell will reach a higher voltage sooner with a constant-voltage system. The advantage of a constant-current system over a constant-voltage system is that it will increase the cell capacity. This is because the cell will be charged at a higher current for a longer period of time with a constant-current system.
The option “avoiding excessive gassing” is not correct. Both constant-current and constant-voltage systems can cause excessive gassing if the cell is not properly charged.