In Iocomotives, Swaying Couple Is Caused Due to

In Iocomotives, swaying couple is caused due to

A) Unbalanced part of the secondary forces acting at a distance between the line of stroke of the cylinders
B) Vertical unbalanced primary forces acting in the plane of the wheels
C) Vertical unbalanced primary forces acting in the plane of the wheels.
D) Unbalanced parts of the primary forces acting at a distance between the line of stroke of the cylinders
E) Horizontal unbalanced primary forces acting in the plane of the wheel