If a plane electromagnetic wave satisfies the equation $\frac{\partial^{2} \mathrm{E}_{\mathrm{x}}}{\partial \mathrm{z}^{2}}=\mathrm{c}^{2} \frac{\partial^{2} \mathrm{E}_{\mathrm{x}}}{\partial \mathrm{t}^{2}}$, the wave propagates in the
- $x$-direction
- $z$-direction
- $y$-direction
- $x y$ plane at an angle of $45^{\circ}$ between the $x$ and $z$ directions