When a series of wheel loads crosses a simply supported girder, the maximum bending moment under any given wheel load occurs when________________?

A. the centre of gravity of the load system is midway between the centre of span and wheel load under consi-deration
B. the centre of span is midway between the centre of gravity of the load system and the wheel load under consideration
C. the wheel load under consideration is midway between the centre of span and the centre of gravity of the load system
D. none of the above