A convex lens `` 'A' `` of focal length `` 20\, cm `` and a concave lens `` 'B' `` of focal length `` 5\, cm `` are kept along the same axis with a distance `` 'd' `` between them. If a parallel beam of light falling on `` 'A' `` leaves `` 'B' `` as a parallel beam, then the distance `` 'd' `` in cm will be :
(A)25
(B)15
(C)50
(D)30