Look at the diagram and think about the action represented by the arrow. The image is two groups of blood vessels one is oxygen-rich blood and another is oxygen-poor blood. These two blood vessels are connected by a network finer blood vessels. There is one arrow from oxygen-rich blood to oxygen-poor blood.

Which body part label should be added to the box in the diagram?

A. veins
B. tissues
C. arteries
D. capillaries