Three parallel plates are placed above each other with equal displacement $\vec{d}$ between neighbouring plates. The electric field between the first pair of the plates is $\vec{E}_1$ and the electric field between the second pair of the plates is $\vec{E}_2$. The potential difference between the third and the first plate is - (A) $$\vec{E}_1 + \vec{E}_2$ \cdot \vec{d}$ (B) $$\vec{E}_1 - \vec{E}_2$ \cdot \vec{d}$ (C) $$\vec{E}_2 - \vec{E}_1$ \cdot \vec{d}$ (D) $\frac{d(E_1 + E_2)}{2}$