The activity of a radioactive substance is $R_1$ at time $t_1$ and $R_2$ at time $t_2$. If $\lambda$ is the decay constant,then which of the following is correct?

  • A
    $R_1 = R_2$
  • B
    $R_1 = R_2 e^{-\lambda(t_1 - t_2)}$
  • C
    $R_1 = R_2 e^{\lambda(t_1 - t_2)}$
  • D
    $R_1 = R_2(t_1 / t_2)$

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