A person travelled from one place to another at an average speed of $40 \,km / h$ and back to the original place at an average speed of $50\, km / h$. What is his average speed in $km / h$ during the entire trip?

  • A

    $45$

  • B

    $20 \sqrt{5}$

  • C

    $400 / 9$

  • D

    $50$

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