The lateral magnification of the lens with an object located at two different positions u1 and u2 are m1 and m2, respectively. Then the focal length of the lens is
(a) (u1m1 - u2m2)/(m1 - m2)
(b) (u1m2 - u2m1)/(m1 - m2)
(c) (u1m1 + u2m2)/(m1 + m2)
(d) (u1m2 + u2m1)/(m1 + m2)