What are C and D in order? What does the looped part represent?

Question

( a ) Vena Cavae, Ventral Aorta; Systemic circulation
( b ) Vena Cavae, Dorsal Aorta; Systemic circulation
( c ) Ventral Aorta, Vena Cavae; Systemic circulation
( d ) Dorsal Aorta, Vena Cavae; Systemic circulation