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

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

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Applied Mechanics and Graphic Statics
The necessary condition of equilibrium of a body is:

Algebraic sum of the moments of the forces about a point must be zero
Algebraic sum of horizontal components of all the forces must be zero
All (A), (B) and (C)
Algebraic sum of vertical components of all the forces must be zero

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