The average of the test scores of a class of $'m'$ students is $70$ and that of $'n'$ students is $91.$ When the scores of both the classes are combined, the average is $80 .$ What is $n / m$ ?

  • A

    $\frac{11}{10}$

  • B

    $\frac{13}{10}$

  • C

    $\frac{10}{13}$

  • D

    $\frac{10}{11}$

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