A small sphere of mass \(m\) is released from rest at \(t = 0\) at the surface of a deep column of viscous fluid. The fluid exerts a resistive drag force on the sphere given by \(\vec{F}_d = -b\vec{v}\), where \(b\) is a positive constant and \(\vec{v}\) is the sphere's velocity. Let the downward direction be defined as positive, with the sphere's release position at \(y = 0\). Which of the following best describes the shape of the graph of the sphere's downward acceleration \(a\) as a function of its downward position \(y\)?