`A` and `B` are two trains moving parallel to each other. If a balls is thrown vertically up from the train `A`, the path of the ball is
A. parabola for an observer standing on the ground
B. vertical straight line for an observer in `B` when `B` is moving with same speed but in same direction
C. a parabola for an observer in `B` when `B` is moving with same speed but in opposite direction.
D. all the above are true
