C. When you take a step, the ground pushes back on your foot with the same amount of force. If it pushed back with less, your foot would go through the floor; if it pushed back with more, it would actually push your foot off the ground. But they're in opposite directions; your foot pushes down, while the ground pushes up.
Hope this helps!
It is referred as <span>parasitism</span>
B. I think, because they are both mechanical. Don't quote me on that <span />
