$A$ radioactive sample is an $\alpha$-emitter with a half-life of $138.6$ days. $A$ student observes its activity to be $2000$ disintegrations per second. The number of radioactive nuclei for this given activity is:

  • A
    $3.45 \times 10^{10}$
  • B
    $1 \times 10^{10}$
  • C
    $3.45 \times 10^{15}$
  • D
    $2.75 \times 10^{11}$

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