An object of mass \(m\) on a horizontal frictionless surface is attached to a non-linear spring that exerts a restoring force \(F(x) = -kx - cx^3\), where \(k\) and \(c\) are positive constants and \(x\) is the displacement from equilibrium. The object is released from rest at position \(x = A\). Which of the following expressions represents the period \(T\) of the resulting oscillation?