Final answer:
The ceiling exerts a force of 100 N on the person, which is equal to the force they exert on it. The floor exerts a force of approximately 790 N on the person, countering their weight and the force they push against the ceiling with.
Step-by-step explanation:
When you stand in a low tunnel and push against the ceiling with a force of 100 N, using Newton's Third Law of Motion, we know that for every action there is an equal and opposite reaction. Therefore, the ceiling exerts a force of 100 N on you, which is option B. As for the force the floor exerts on you, since you are in a state of static equilibrium, this force must counteract both your weight and the force you apply to the ceiling. Therefore, the force exerted by the floor on you is the sum of your weight (due to gravity) and the force you exert upward. The gravitational force (weight) on you is calculated as the mass (70 kg) times the acceleration due to gravity (9.8 m/s2), which is 70 kg × 9.8 m/s2 = 686 N approximately. When added to the force you're exerting (100 N), the total force exerted by the floor is approximately 790 N, which is option D.