$A$ radioactive sample consists of two distinct species having an equal number of atoms $N_0$ initially. The mean-life of one species is $\tau$ and of the other is $5\tau$. The decay products in both cases are stable. The total number of radioactive nuclei at $t = 5\tau$ is:

  • A
    $N_0 \left( \frac{e^5 + 1}{e^5} \right)$
  • B
    $N_0 \left( \frac{e^4 + 1}{e^5} \right)$
  • C
    $N_0 \left( \frac{e + e^5}{e^5} \right)$
  • D
    $N_0 e^{-3}$

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