A long cylindrical wire of radius \(R\) carries a total current with a nonuniform current density directed parallel to the axis of the wire. The magnitude of the current density as a function of radial distance \(r\) from the central axis is given by \(J(r) = J_0 \left(1 - \dfrac{r}{R}\right)\) for \(0 \le r \le R\), where \(J_0\) is a constant. Which of the following expressions represents the total current \(I\) passing through a cross section of the wire?