Answer:
In this case the gravitational force is conservative, but the force you exert is not. So, as you lift the box, the work done by non-conservative forces (you) is not zero, and as a result, the total energy of the box increases.
But, Lifting a box is not scientifically considered "work"
Explanation: You do no work against gravity because you are not moving any distance (more accurately displacement) in the direction of the force i.e. the force and displacement are perpendicular once you have lifted the boxes. As work done = force x displacement parallel to the force, no work is done.
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