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

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

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Applied Mechanics and Graphic Statics
The phenomenon of collision of two elastic bodies takes place because bodies

Immediately after collision come momentarily to rest
Tend to compress each other till they are compressed maximum possible
All listed here
Attempt to regain its original shape due to their elasticities

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