The activity of a radioactive substance is $R_1$ at time $t_1$ and $R_2$ at a later time $t_2$. Its decay constant is $\lambda$. Which of the following relations is correct?

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