A single-turn circular loop of flexible wire lies in the plane of the page in a uniform, constant magnetic field of magnitude \(B\) directed into the page. At time \(t = 0\), the loop has an initial radius \(r_0\) and begins expanding radially outward in the plane such that its radius as a function of time is \(r(t) = r_0 + v_0 t\), where \(v_0\) is a constant speed. Which of the following expressions correctly gives the magnitude of the instantaneous electromotive force \(\varepsilon(t)\) induced in the loop as a function of time \(t\)?