A block of mass \(m\) rests on a horizontal, frictionless surface and is attached to a non-linear spring. The spring exerts a restoring force given by \(F(x) = -kx - \beta x^3\), where \(k\) and \(\beta\) are positive constants and \(x\) is the displacement of the block from its equilibrium position at \(x = 0\). The block is pulled to a displacement \(x = d\) and released from rest. Which of the following expressions gives the maximum speed \(v_{\text{max}}\) of the block, and what is the limiting value of \(v_{\text{max}}\) as \(\beta \to 0\)?