A positively charged particle moves with velocity \(\vec{v}\) in a region containing a non-uniform magnetic field \(\vec{B}\) aligned generally along the \(z\)-axis (a magnetic bottle configuration). Near the "neck" of the bottle at \(z > 0\), the magnetic field lines converge toward the \(z\)-axis, introducing a radial magnetic field component \(B_r < 0\) directed toward the axis. The particle spirals around the \(z\)-axis with an azimuthal velocity component \(v_\theta\) while traveling in the \(+z\)-direction with axial speed \(v_z\). As the particle approaches the neck, its axial speed \(v_z\) decreases to zero and the particle reflects back toward \(z = 0\). Which of the following explanations correctly accounts for the axial deceleration of the particle and the work done by the magnetic field during this reflection?