A rectangular tank of mass $\;m_{0}\;$ and charge Q over it is placed over a smooth horizontal floor. A horizontal electric field E exist in the region . Rain drops are falling vertically in the tank at the constant rate of n drops per second . Mass of each drop is m . find velocity of tank as function of time .

Question

$(a)\;\large\frac{2QEt}{m_{0}+m n t}\qquad(b)\;\large\frac{QEt}{m_{0}+m n t}\qquad(c)\;\large\frac{QEt}{2(m_{0}+m n t)}\qquad(d)\;None$