The potential energy in food is what we call a Calorie, which (note the capitalization) is actually a kiloCalorie (1000 calories). Just pick any food high in calories; fats and oils are very calorie dense.
B, the independent variable because this is the one that is purposefully changed in an experiment to test the hypothesis.
Some advances is that organelles, cells, and the difference between the cells of a plant and a cell of an animal. This was an advance because not only did we discover new things but we learned how or body functions with them and how they work. Hopefully you got what you were looking for. :)
