Theory of Machine
The example of rolling pair is

Ball bearing and roller bearing
Lead screw of a lathe
Bolt and nut
Ball and socket joint

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Theory of Machine
In order to give a complete secondary balance of a multi-cylinder inline engine,

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

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