The thymus gland is a lobular structure located on the $A$ side of the aorta and plays a significant role in the development of the $B$ system.
Choose the correct combination of $A$ and $B$.

• A
  $A$-ventral,$B$-immune
• B
  $A$-lateral,$B$-circulatory
• C
  $A$-dorsal,$B$-immune
• D
  $A$-dorsal,$B$-circulatory