Which statement accurately describes a balanced force?

a. A 25-N force acts on the left side of the marble at the same time as a 25-N force acts on the right side of the marble.

b. A marble rolling downhill suddenly comes to rest at a garden wall.

c. A 2-N force presses down on the marble at the same time as a 1.5-N force presses up on the marble.

d. A marble resting on the sidewalk suddenly begins to roll downhill without being touched.