Final answer:
A student standing at the top of a step stool does not move downward because the step stool provides an equal and opposite force against gravity, and the student exerts an upward force equal to their gravitational force, resulting in balanced forces and no movement.
Step-by-step explanation:
The student standing at the top of a step stool does not move downward because of two reasons. First, the step stool exerts a force on the student in the opposite direction of gravity. This is described by Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. Secondly, Newton's second law helps us understand that the forces on the student are balanced since the net force is zero.
It is important to note that the two correct statements explaining why the student does not move downward are: B) The step stool exerts a force on the student in the opposite direction of gravity, and C) The student exerts an upward force on the step stool equal to the gravitational force. The other options, A) and D), are incorrect because mass is always affected by gravity regardless of its size and a step stool cannot block the gravitational force.