$P$ is the midpoint of edge $A B$, $Q$ is the midpoint of edge $C D$, then the length of line $P Q$ is ( ).
(A ) $\frac{1}{2}$.
(B) $\frac{\sqrt{3}}{2}$.
(C ) $\frac{3}{4}$.
(D) $\frac{\sqrt{2}}{2}$.
(1987, North High School Grade 1)
Solve for the Chengxi body $\triangle B C$ D

a regular hexahedron (cube)

cube, as shown in Figure 7, PQ
the length is the diagonal of the cube's face, which
distance, that is, the side length of the cube is $\frac{\sqrt{2}}{2}$. Therefore, the answer is (D).