[Closed] position costrain

Hi
Is this possible by scripting
We have A B C D objects B and D are above
I like to get, Until A reached at B position C follow A as son as A reached at B position C get D position and come down by D position

The simple script in script position on C is
dist= distance A B
if dist<=xxxx then A else D

It works fine but when dist get smaller than xxxx C get position back to A I like C get stick on D

Another solution is link constrain can we do sth like this with link constrain of course not in animation sth like wiring whenever A get B … not at specific time and key

Thanks in advance