Completely unrelated, but that is a great sentence, I want that painted as a quote on my wall.
Also, the punctiation seems right to me. But you might be able to use a ";" instead of a comma, im gonna check that real quick.
Answer:
B.
Explanation:
B. Dog 1's position is Dog 2's position reflected across the x-axis; only the signs of the y-coordinates of Dog 1 and Dog 2 are different.