Answer: A. Newton’s third law explains that for every action there must be a reaction pair. Even a box sitting on the floor can be explained by Newton's third law. This is why (b) and (c) are incorrect. An example of Newton’s third law is normal force: A pencil sitting on a table pushes down on that table. The 3rd law tells us that the table is pushing back up on the pencil. Another example is when two cars collide — they exert equal and opposite forces upon collision irrelevant of their masses.