In a radioactive material,the activity at time $t_{1}$ is $R_{1}$ and at a later time $t_{2}$,it is $R_{2}$. If the decay constant of the material is $\lambda$,then:

  • A
    $R_{1}=R_{2} e^{-\lambda\left(t_{1}-t_{2}\right)}$
  • B
    $R_{1}=R_{2} e^{\lambda\left(t_{1}-t_{2}\right)}$
  • C
    $R_{1}=R_{2}\left(t_{2} / t_{1}\right)$
  • D
    $R_{1}=R_{2}$

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