18) Potential energy of a system comprising of point charges U₁. When  a charge q  is added in the system without disturbing other charges, the potential energy becomes U₂. The potential of the point where the charge q is placed in the system is.

$$\begin{gathered} \left(A\right) \frac{U_2-U_1}{q} \\ \left(B\right) \frac{U_1-U_2}{q} \\ \left(C\right) \frac{U_1+U_2}{2q} \\ \left(D\right) \frac{U_2-U_1}{2q} \end{gathered}$$