Theory of Machine
In order to give the primary balance of the reciprocating parts of a multi-cylinder inline engines

Both (A) and (B)
The algebraic sum of the primary forces must be equal to zero
The algebraic sum of the couples about any point in the plane of the primary forces must be equal to zero
None of these

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Theory of Machine
Kinematic pairs are those which have

Elements of pairs not held together mechanically
Point or line contact between the two elements when in motion
Two elements that permit relative motion
Surface contact between the two elements when in motion

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