$A$ capacitor of unknown capacitance $C$ is connected across a battery of $V$ volt. The charge stored in it becomes $Q$ coulomb. When the potential across the capacitor is reduced by $V^{\prime}$ volt,the charge stored in it becomes $Q^{\prime}$ coulomb. The capacitance $C$ is:

• A
  $\frac{Q-Q^{\prime}}{\sqrt{V^{\prime}}}$
• B
  $\frac{V^{\prime}}{Q-Q^{\prime}}$
• C
  $\frac{Q+Q^{\prime}}{V^{\prime}}$
• D
  $\frac{Q-Q^{\prime}}{V^{\prime}}$