Hmm thanks but I only understood from that how to do it on positions .
No Idea how to do it with opacity .
(I dont have like the doted road)
Why don't you try USING it on other properties before you give up on it?
Ok thanks this is what I did and it works :
Selected A and B
Pressed on one of them with the right side of the mouse
Slected Keyframe Interpolation - Temporal Interpolation - HOLD