$A$ certain radioactive element disintegrates with a decay constant of $7.9 \times 10^{-10} / s$. At a given instant of time, if the activity of the sample is equal to $55.3 \times 10^{11}$ disintegration/second, then the number of nuclei at that instant of time is:
- A$7.0 \times 10^{21}$
- B$4.27 \times 10^{13}$
- C$4.27 \times 10^3$
- D$6 \times 10^{23}$
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