An object of mass \(m\) is dropped from rest and falls vertically through a fluid. The motion of the object is governed by the differential equation \(m \dfrac{dv}{dt} = mg - bv\), where \(v\) is the speed of the object, \(g\) is the acceleration due to gravity, and \(b\) is a positive drag constant. Which of the following expressions represents the terminal speed \(v_T\) of the object?