A light ray travelling parallel to the principal axis of a concave mirror strikes the mirror at angle of incidence `theta`. If radius of curvature is `R`, then after reflection, the ray meets the principal axis at a distance d from the centre of curvature then d is
A. `(R)/(2)`
B. `(Rcostheta)/(2)`
C. `(R)/(2costheta)`
D. `(R)/(2)(1+costheta)`