Correct Response: B. Because a triangle is the simplest geometric figure that will not change shape when the lengths of its sides are fixed (due to the side-side-side theorem of congruent triangles), the triangle is the basic structural unit of a truss bridge. Circles (A) and cylinders (D) can also be very stable shapes. But they do not lend themselves as well to truss constructions, which typically consist of a lattice of straight beams. Squares (C) that are not reinforced by triangles result in bending forces at the nodes of the lattices due to the fact that their angles are not geometrically constrained. Hence, they are not as stable as triangles.
