$A$ $10\, mm$ long awl pin is placed vertically in front of a concave mirror. $A$ $5\, mm$ long image of the awl pin is formed at $30\, cm$ in front of the mirror. The focal length of this mirror is ......... $cm$.

  • A
    $-30$
  • B
    $-20$
  • C
    $-40$
  • D
    $-60$

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