In a radioactive material the activity at time $t_1$ is $R_1$ and at a later time $t_2$, it is$R_1$. If the decay constant of the material is $\lambda$, then

Question

( 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 ) $R_1=R_2 (t_2/t_1)$