A fox sits on a box which is pulled by a massless rope. The rope has tension of constant magnitude T$,$ the mass of the fox is m$,$ and the mass of the box is M$.$ The rope makes an angle with the horizontal and there is a coefficient of kinetic friction of k between the box and the ground. The box goes on to slide a distance d$,$ and during this displacement the fox remains stationary relative to the box $($in other words, it does not slip against the box$).$

For the fox and box, what coordinate system should be chosen and why?