Which of the following statements are correct?
1. 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 the gears.
2. For involute gears the pressure angle changes with change in centre distance between gear.
3. The velocity ratio of compound gear train depends upon the number of teeth of the input and output gears only
4. Epicyclic gear trains involve rotation of at least one gear axis about some other gear axis
(a) 1,2and 3 (b) 1 and 3
(c) 1,2 and 4 (d) 2,3 and 4