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. `(y^(2) - x^(2))/(2y)`
B. `(y^(2) + x^(2))/(2y)`
C. `y-x`
D. `(y^(2) + x^(2))/(y-x)`
