Okay, let's think this through step-by-step:
* A to D is 1 km
* B to C is 2 km
* B to D is 3 km
* A to B is 4 km
* C to D is 5 km
So we have:
A -> B = 4 km
B -> C = 2 km
C -> D = 5 km
We want to find A -> C.
A -> B is 4 km
B -> C is 2 km
So A -> C = 4 + 2 = 6 km
Therefore, the distance between stops A and C is 6 km.