What should be the activity of a radio active sample of mass ‘m’ and decay constant  $\mathrm{\lambda }$, after time t ? (Take molecular mass of sample be M and Avogardro’s number be ${\mathrm{N}}_{\mathrm{a}}$).

• A

  $\mathrm{A}=\left(\frac{\mathrm{MNa\lambda }}{\mathrm{t}}\right){\mathrm{e}}^{-\mathrm{\lambda t}}$

• B

  $\mathrm{A}=\left(\frac{\mathrm{mNa\lambda }}{\mathrm{M}}\right){\mathrm{e}}^{-\mathrm{\lambda t}}$

• C

  $\mathrm{A}=\left(\frac{\mathrm{mNa\lambda }}{\mathrm{t}}\right){\mathrm{e}}^{-\mathrm{\lambda t}}$

• D

  $\mathrm{A}=\left(\frac{\mathrm{Mm\lambda }}{\mathrm{Na}}\right){\mathrm{e}}^{-\mathrm{\lambda t}}$