The $\left(x,y\right)$ coordinates of the corners of a square plate are $\left(0,0\right),\left(L,0\right),\left(L,L\right)$ and $\left(0,L\right)$. The edges of the plate are clamped and traverse standing waves are setup in it. If $u\left(x,y\right)$ denotes the displacement of the plate at the point $\left(x,y\right)$ at some instant of time, the possible expression$\left(s\right)$ for $u$ is $\left(are\right) \left(a=positive cons\tan t\right)$