Applied Mechanics and Graphic Statics
If the given forces P₁, P₂, P₃ and P₄ are such that the force polygon does not close, then the system will

Always reduce to a resultant force
Be in equilibrium
Both (A) and (C)
Always reduce to a couple

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Applied Mechanics and Graphic Statics
The characteristic of a couple is:

Algebraic sum of moments of forces, constituting a couple, about any point, is same
A couple can be never the balanced by a single force
Algebraic sum of forces, constituting a couple is zero
All listed here

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