Theory of Machine
When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of the other, the pair formed is known as a

Self-closed pair
Higher pair
Lower pair
Force-closed pair

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Theory of Machine
In a mechanism, the fixed instantaneous centers are those centers which

None of the listed here
Moves as the mechanism moves, but joints are of permanent nature
Remain in the same place for all configurations of the mechanism
Vary with the configuration of the mechanism

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