A spring has unstretched length L, and the middle of the spring is marked with a red dot. The spring has force constant k. We extend the spring by a distance x by exerting a force kx on the right-hand end of the spring. Consider the right-hand half of the spring for a moment. It feels the force kx to its right and it must also feel a force to its left at the red dot.
B does more work when force is the same.
Very nicely seen on a F vs. x plot. Same stretching, larger slope does more work. In picking up a book from the floor and putting it on a table you do work. However, the initial and final velocities are the same, so there is no change in the book's kinetic energy. Explain why or why not.