A particle of mass \(m\) and positive charge \(q\) enters a uniform magnetic field \(\vec{B} = B_0 \hat{k}\) with an initial velocity \(\vec{v}_0\) directed at an angle \(\theta_0\) (where \(0 < \theta_0 < \pi/2\)) relative to the positive \(z\)-axis. In addition to the magnetic force, the particle experiences a linear drag force \(\vec{F}_D = -b \vec{v}\), where \(b\) is a positive constant. Which of the following pairs correctly describes the pitch angle \(\theta(t)\) (the angle between \(\vec{v}\) and \(\vec{B}\)) and the radius \(R(t)\) of the particle's helical path as functions of time \(t\)?