An object is placed on the principal axis of a convex mirror. Distance of onject from the mirror is `40cm`. A plane mirror is placed between the object and the convex mirror, covering lower half below principal axis of the mirror. Distance between the object and the plane mirror is `30cm`. If there is no parallax between the two images formed by plane mirror and convex mirror, the focal length of the convex mirror is
A. `20cm`
B. `40cm`
C. `60cm`
D. `80cm`