Carla has been working out and is trying to show her friends how much work she can do. In which of the following situations would she not be doing work? A. If she pushed a very large mass across a room. B. If she carried heavy boxes up a long flight of stairs. C. If she held a very heavy pair of weights in front of her. D. If she lifted a heavy barbell over her head and lowered it back down to the floor several times.

Option C) if she held a very heavy pair of weights in front of her

Explanation:

Work is done when force is applied over a distance. i.e Workdone = Force X Distance

However, Since Carla's applies force to hold the heavy pair of weights BUT no distance is being covered due to the static (non-moving) posture she took, no work is done at that moment in time.

Thus, option C is the answer

not always but somethings can be reproduced in a laboratory.