6 Let $x, y, r \in \mathbf{R}$, then the moving point $(x, y)$ that makes the algebraic expression $\sqrt{r^{2}-x^{2}-y^{2}}+\lg (x+y)$ meaningful forms the figure ( ).
(A) symmetric about the $x$-axis
(B) symmetric about the $y$-axis
(C) symmetric about the line $y=x$
(D) symmetric about the line $y=-x$
(6) $\mathrm{C}$ Hint: From the problem, we have $\left\{\begin{array}{l}x^{2}+y^{2} \leqslant r^{2}, \\ x+y>0,\end{array}\right.$ then the figure formed by the moving point $(x, y)$ is symmetric about the line $y=x$.