In Problems 1 - 12, a differential equation is given along with the field or problem area in which it arises. Classify each as an ordinary differential equation (ODE) or a partial differential equation (PDE), give the order, and indicate the independent and dependent variables. If the equation is an ordinary differential equation, indicate whether the equation is linear or nonlinear.
$$\begin{array}{l}{\frac{d^{2} y}{d x^{2}}-0.1\left(1-y^{2}\right) \frac{d y}{d x}+9 y=0} \\ {\text { (van der Pol's equation, triode vacuum tube) }}\end{array}$$