The focal length of a lens of dispersive power $0.45$ which should be placed in contact with a convex lens of focal length $84\;cm$ and dispersive power $0.21$ to make the achromatic combination from the two lenses, in cm is :

Question

\[\begin {array} {1 1} (a)\;45 & \quad (b)\;90 \\ (c)\;180 & \quad (d)\;-180 \end {array}\]