Circles ${x^2} + {y^2} + 2gx + 2fy = 0$ and ${x^2} + {y^2}$ $ + 2g'x + 2f'y = $ $0$ touch externally, if

  • A

    $f'g = g'f$

  • B

    $fg = f'g'$

  • C

    $f'g' + fg = 0$

  • D

    $f'g + g'f = 0$

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  • [IIT 1994]