One of the following equations is an equivalent representation of the wave equation. Which one? \begin{array}{l} \bigcirc\left(\frac{\partial}{\partial t}+c \frac{\partial}{\partial x}\right)\left(\frac{\partial}{\partial t}-c \frac{\partial}{\partial x}\right) u=0 \\ \bigcirc\left(\frac{\partial}{\partial t}+c \frac{\partial}{\partial x}\right)\left(\frac{\partial}{\partial t}+c \frac{\partial}{\partial x}\right) u=0 \\ \bigcirc\left(\frac{\partial u}{\partial t}+c \frac{\partial u}{\partial x}\right)^{2}=0 \\ \bigcirc\left(\frac{\partial u}{\partial t}+c \frac{\partial u}{\partial x}\right)\left(\frac{\partial u}{\partial t}-c \frac{\partial u}{\partial x}\right)=0 \\ \bigcirc\left(\frac{\partial u}{\partial t}+c \frac{\partial u}{\partial x}\right)\left(\frac{\partial u}{\partial t}+c \frac{\partial u}{\partial x}\right)=0 \end{array}

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