For a radioactive element,the mean life is $\tau$. At time $t = 0$,the number of nuclei decaying per unit time is $n$. The number of nuclei that have decayed between time $0$ and $t$ is:

  • A
    $n\tau {e^{ - \frac{t}{\tau }}}$
  • B
    $n\left( {1 - {e^{\frac{t}{\tau }}}} \right)$
  • C
    $n\tau \left( {1 - {e^{ - \frac{t}{\tau }}}} \right)$
  • D
    $n{e^{ - \frac{t}{\tau }}}$

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