In the following partial differential equation, $\theta$ is a function of $t$ and $z$, and $D$ and $K$ are functions of \$\theta$$D(\theta)\frac{\partial^2\theta}{\partial z^2}+\frac{\partial K(\theta)}{\partial z}-\frac{\partial\theta}{\partial t}=0$$

The above equation is

1. a second order linear equation
2. a second degree linear equation
3. a second order non-linear equation
4. a second degree non-linear equation

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