$A$ doctor uses a stethoscope to hear the sounds produced during each cardiac cycle. When is the second heart sound heard?

• A
  When the $AV$ node receives signals from the $SA$ node.
• B
  When the $AV$ valves open.
• C
  When the ventricular walls vibrate as blood leaves the atria.
• D
  When the semilunar valves close as blood enters the arteries from the ventricles.