The correct answer is:
B. They act on different objects, so they would not appear together.
Let's take for example a box sitting on a table. If we draw the free-body diagram of the box, we would draw two forces:
- the weight of the box (downward)
- the normal reaction of the table on the box (upward)
However, this is not a pair of action-reaction forces as described in Newton's third law. In fact, the correct action-reaction pair would be:
- weight of the box (=gravitational force exerted by the Earth on the box)
- gravitational force exerted by the box on the Earth
We see that these two forces act on two different objects (the first one on the box, the second one on the Earth), so they do not appear in the same free-body diagram.
