Just the answer is fine:)

Consider the parametric surface S given by r(u, v) = (u cos v, u sin v, v). If the normal vector of the tangent plane to S at the point P(1/2, V3/2, 7/3) is orthogonal to the vector v = (0, a, 2), then the constant a equals Select one: O a. 3 cross out O b. 2 cross out cross out O c. 1 cross out O d. 0 cross out O e. 4