A block of mass \(m\) slides along a horizontal frictionless track and enters a medium containing a viscous fluid at time \(t = 0\) and position \(x = 0\) with an initial velocity \(v_0\) directed in the \(+x\)-direction.

Within the fluid, the block experiences a resistive drag force given by \(\vec{F} = -b\vec{v}\), where \(b\) is a positive constant and \(\vec{v}\) is the instantaneous velocity of the block.

Which of the following graphs best represents the horizontal acceleration \(a_x\) of the block as a function of its position \(x\) from \(x = 0\) until the block comes to rest?