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 $O$ formed by $M_1$ and $M_2$ coincide. The magnitude of $f$ is

  • A
    $\frac{{{x^2} - {y^2}}}{{2y}}$
  • B
    $\frac{{{x^2} + {y^2}}}{{2y}}$
  • C
    $x - y$
  • D
    $\frac{{{x^2} + {y^2}}}{{x - y}}$

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