Rachel is pushing a box across the floor while Jon, at the same time, is hoping to stop the box by pushing in the opposite direction. There is friction between the box and floor. If the box is moving at constant speed, then the magnitude of Rachel's pushing force is

A. Greater than the magnitude of Jon's force. 

B. Equal to the magnitude of Jon's force. 

C. Less than the magnitude of Jon's force. 

D. The question can't be answered without knowing how large the friction force is.