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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