Two different radioactive elements with half-lives $T_1$ and $T_2$ have undecayed atoms $N_1$ and $N_2$ respectively, present at a given instant. The ratio of their activities at this instant is

  • A
    $N_1T_1/N_2T_2$
  • B
    $N_1T_2/N_2T_1$
  • C
    $N_1T_2/T_1T_2$
  • D
    $T_1T_2/N_1N_2$

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