If f(x) is monotonic and differentiable function, then `2int_f(a)^f(b) x(b-f^-1(x))dx` equals to
(A)`int_a^b x(f^2(x)-f^2(a))dx` (B)`int_a^b (f(x)-f(a))dx`
(C)`f^2(x)-f^2(a)` (D) None of these