newtonian mechanics - Work done by action-reaction pair of tension

Work done by action-reaction pair of tension force on a system is zero if the string is inextensible I am trying to understand this statement taking the following arrangement in consideration. He

Trouble with the concept of tension.

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Where's the action force if the normal force is a reaction force?

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Work done by tension in each string for two accelerating blocks. Work done by constant force.

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