A student tries to push a refrigerator-sized box of textbooks safely across the classroom. The box does not move. He asks for help from other students.

The box starts to move when the number of students shown in the image is pushing together.

Based on this information, what conclusion can be drawn?

The box has a mass greater than the combined mass of the first two students pushing.

The forces acting on the box became unbalanced when the third student started pushing.

The only force acting on the box is the push applied by the students.

When the third student started to push, the box’s mass decreased