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$\left[ \frac{N_A}{m} M_w \right] \lambda$
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