Find f' (x) and find the value(s) of x where the tangent line is horizontal. f(x) = x (x-13)7 f' (x) = x(x-13) (7x +6(x - 13)) Select the correct choice below and, if necessary, fill in the answer box within your choice. O A. The tangent line is horizontal at x = 0,6,13 (Use a comma to separate answers as needed.) O B. The tangent line is never horizontal