12. A series of 3 uncharged concentric spherical conducting shells surround a small central charge q. The potential at a point outside the third shell, at distance r from the center, and relative to V = 0 at ∞, is (A) −3keq/r (B) +3keq/r (C) − (ln 3)keq/r (D) + (ln 3)keq/r (E) +keq/r