Solution
= Solution
{c}
Use the two-dimensional <streamfunction> convention $u=\psi_y$, $v=-\psi_x$. Here
$$
u=-\sin x\sin y,\qquad v=-\cos x\cos y.
$$
The <divergence> is $u_x+v_y=-\cos x\sin y+\cos x\sin y=0$, so \b[the flow is incompressible].
= Solution
{c}
Use the two-dimensional <streamfunction> convention $u=\psi_y$, $v=-\psi_x$. Here
$$
u=-\sin x\sin y,\qquad v=-\cos x\cos y.
$$
The <divergence> is $u_x+v_y=-\cos x\sin y+\cos x\sin y=0$, so \b[the flow is incompressible].