The atomic weight of a radioactive element is $M_w$ grams. The radioactivity of $m$ grams of this element is ......... ($N_A$ is Avogadro's number,$\lambda$ is the decay constant).

• A
  $N_A \lambda$
• B
  $\left[ \frac{N_A}{M_w} m \right] \lambda$
• C
  $\left[ \frac{N_A}{m} \right] \lambda$
• D
  $\left[ \frac{N_A}{m} M_w \right] \lambda$