A long solenoid of length \(L_0\) has a total of \(N\) turns non-uniformly distributed along its length such that the turn density is given by \(n(x) = c x\) for \(0 \le x \le L_0\), where \(c\) is a constant and \(x\) is measured from one end. The solenoid carries a constant current \(I\). The solenoid is then uniformly compressed along its length to a new length of \(\dfrac{L_0}{2}\) while maintaining the same total current \(I\). Which of the following correctly gives the magnetic field magnitude \(B\) at the midpoint of the solenoid before compression and the factor by which the magnetic field magnitude at the midpoint changes after compression?