$23 \cdot 38$ As shown, $B, C, D$ are squares with side length 1, $A, E, F$ are isosceles right triangles, $G$ is an equilateral triangle, and this figure can be folded along the lines in the diagram to form a polyhedron with these polygons as faces. What is the volume of this polyhedron?
(A) $\frac{1}{2}$.
(B) $\frac{2}{3}$.
(C) $\frac{3}{4}$.
(D) $\frac{5}{6}$.
(E) $\frac{4}{3}$.
(48th American High School Mathematics Examination, 1997)
[Solution] The given figure can be folded into a square with a corner removed, this corner being a triangular pyramid, whose volume is
$$
V_{\text {triangular pyramid }}=\frac{1}{3} \cdot \frac{1}{2} \cdot 1 \cdot 1 \cdot 1=\frac{1}{6} \text {. }
$$

Therefore, $V=V_{\text {cube }}-V_{\text {triangular pyramid }}=\frac{5}{6}$, so the answer is $(D)$.