A bell tent consists of a conical portion above a cylindrical portion placed on the ground. For a given volume and a circular base of a given radius, the amount of the canvasused is a minimum when the semi-vertical angle of the cone is
(A) $\cos ^{-1} 2 / 3$
(B) $\sin ^{-1} 2 / 3$
(C) $\cos ^{-1} 1 / 3$
(D) None of these