A 2-dimensional resonant cavity supports a TM mode built from a function

\(\psi(x, y, t)=\sin \left(\vec{k}_a \cdot \vec{r}-\omega t\right)+\sin \left(\vec{k}_b \cdot \vec{r}-\omega t\right)+\sin \left(\vec{k}_a \cdot \vec{r}+\omega t\right)+\sin \left(\vec{k}_b \cdot \vec{r}+\omega t\right)\)

where\(\vec{k}_a\) and \(\vec{k}_b\), lie in the xy-plane and make angles \(\frac{\pi}{4}\) and \( \frac{3 \pi}{4}\) with the x-axis, respectively. \(0<\left|\vec{k}_a\right|<\left|\vec{k}_b\right|\), then which of the following closely describes the outline of the cavity?