A small sphere of mass \(m\) is released from rest in a fluid that exerts an upward drag force of magnitude \(F_D = b v^2\), where \(b\) is a positive constant and \(v\) is the downward speed of the sphere. Which of the following graphs best represents the sphere's downward acceleration \(a\) as a function of its downward speed \(v\) from release until it reaches terminal velocity \(v_T\)?