6. As shown in Figure 3, there are 81 uniformly spaced grid points, including the boundary points, in a square. $P$ is the center of the square. If a point $Q$ is randomly chosen from the remaining 80 grid points, the probability that the line $P Q$ is a line of symmetry of the square is ( ).
(A) $\frac{1}{5}$
(B) $\frac{1}{4}$
(C) $\frac{2}{5}$
(D) $\frac{9}{20}$
(E) $\frac{1}{2}$
6. C.

It is known that a square has 4 lines of symmetry. Among these four lines, each line has 8 points $Q$ that meet the requirements, so the required probability is $\frac{8 \times 4}{80}=\frac{2}{5}$.