A radioactive sodium isotope has a half-life of $15\ \text{h}$. The table shows experimental data illustrating how its decay rate changes with time. These data do not have the background count rate deducted. What is the background radiation count rate?
- A$12\ \text{counts s}^{-1}$
- B$15\ \text{counts s}^{-1}$
- C$20\ \text{counts s}^{-1}$
- D$30\ \text{counts s}^{-1}$