If `alpha and beta` are eccentric angles of the ends of a focal chord of the ellipse `x^2/a^2 + y^2/b^2 =1`, then `tan alpha/2 .tan beta/2` is (A) `(1-e)/(1+e)` (B) `(e+1)/(e-1)` (C) `(e-1)/(e+1)` (D) none of these