A soap bubble in vacuum has a radius $3\, cm$ and another soap bubble in vacuum has radius $4\, cm$. If two bubbles coalesce under isothermal condition. Then the radius of the new bubble will be .............. $\mathrm{cm}$

  • A

    $7$

  • B

    $5$

  • C

    $4.5$

  • D

    $2.3$

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