Radioactivity — practice question

The diagram illustrates a decay curve for a radioactive substance. From the curve shown, what is the background radiation count?

• A$40\ \text{counts s}^{-1}$
• B$20\ \text{counts s}^{-1}$
• C$5\ \text{counts s}^{-1}$
• D$0\ \text{counts s}^{-1}$