An electric kettle has two coils. When one of these is switched on, the water in the kettle boils in $6\,\min$ . When the other coil is switched on, the water boils in $3\,\min$. If the two coils are connected in series, the time taken to boil the water in the kettle is ............. $min$

  • A

    $3$

  • B

    $6$

  • C

    $2$

  • D

    $9$

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