Choice A Error

It is clearly wrong because a geometrically variable system may also have redundant constraints when the constraints are improperly arranged, so the computational degrees of freedom may be less than zero even for a geometrically variable system.

Expanded:

Rules for the composition of geometrically invariant systems

1. Real and imaginary hinges: the restraining action of two non***ing linear links is equivalent to that of a single hinge. The hinge formed by the two chain rods intersecting at one point is the real hinge. The extension of the two links intersect at a point, the restraining effect is equivalent to the restraining effect of a single hinge at that point, this hinge is called the virtual hinge or instantaneous hinge.

2. Binary rule: two not *** line of the chain at one end of the hinge and constitute a node, called binary.

3. Duality rule: adding a duality or removing a duality in a system does not affect the geometric invariance or geometric variability of the system.

4. Rule of two rigid pieces: two rigid pieces connected by a hinge and a chain rod, and the chain rod does not pass through the hinge, the system is geometrically invariant system, and there is no unnecessary constraints. Two rigid pieces linked with three links, and the three links are not all parallel or not all intersect at one point, the system is geometrically invariant system, and there is no unnecessary constraints.

5. Three rigid pieces rule: three rigid pieces are connected by three hinges, and the three hinges are not in the same straight line, then the system is geometrically invariant system, and there is no unnecessary constraint.

6. Expansion of the rigid piece: Starting from the basic rigid piece, apply the basic rules of composition to form a geometrically invariant system without unnecessary constraints, forming an expanded basic rigid piece.