A long, ideal solenoid of radius \(R\) has \(n\) turns per unit length. The solenoid carries a time-varying current \(I(t)\) that increases at a constant rate \(\dfrac{dI}{dt} = C\), where \(C > 0\). Which of the following expressions gives the magnitude of the induced electric field \(E\) at a distance \(r\) from the central axis of the solenoid, where \(r < R\)?