An isolated metal sphere of radius R, carrying an electric charge Q, is situated in the medium of relative permitivity, E^r. A test charge is placed at a point p, distance r from the surface of the sphere. Let E^o represent the permitivity of free space. The electric potential at p is given by the expression

A. \(\frac{Q}{4 \pi E_o E_r}\)
B. \(\frac{Q}{4 \pi E_o E_r (R +r)}\)
C. \(\frac{Q}{4 \pi E_o E_r(R - r)}\)
D. \(\frac{Q}{4 \pi E_o E_rR}\)  

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Correct Answer: D