[SPM Physics 2020 MRSM ] Why the answer can't be C?

For work to be done, W = F x s, where s is the displacement of the force.

For both A and B, the ‘object’ where force is acted upon does not move, and hence s = 0 and W = 0.

For C, the wood plank rests stationary on top of the worker, and only the worker is walking - there is no FORCE being acted upon on the wood plank other than the normal force of the shoulder on the wood plank - and there is no vertical movement in this case. Hence, no work is done on the wood plank.

Answer is D. Pushing up a box, as there is displacement in the direction of the force applied on the box.

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Ouhhh I see Thank you very much! :grinning_face_with_smiling_eyes::+1::+1: