An object O is placed in front of a small plane mirror `M_(1)` and a large convex mirror `M_(2)` of focal length `f`. The distance between O and `M_(1)` is x, and the distance between `M_(1)` and `M_(2)` is y. The images of O forned by `M_(1)` and `M_(2)` coincide. The magnitude of f is
A. `x-y`
B. `(x^(2)-y^(2))/(2y)`
C. `(x^(2)+y^(2))/(2y)`
D. `(x^(2)+y^(2))/(x-y)`
