The count rate of a Geiger-Muller counter for the radiation of a radioactive material with a half-life of $30\, minutes$ decreases to $5\, s^{-1}$ after $2\, hours$. The initial count rate was..........$s^{-1}$.

  • A
    $25$
  • B
    $80$
  • C
    $625$
  • D
    $20$

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