Which of the following statement is correct?

A. For constant velocity ratio transmission between two gears, the common normal at the point of contact must always pass through a fixed point on the line joining the centers of rotation of gears.

B. For involute gears, the pressure angle changes with the change in center distance between gears.

C. The epicyclic gear trains involve rotation of at least one gear axis about some other gear axis.

D. All of the above

Answer: Option D