The human heart lies in the space present between the two lungs,which is called . . . . . . .

All vertebrates possess a ...$A$.... Fishes have a ...$B$... chambered heart with atrium and ventricles. Amphibians and reptiles have a ...$C$... chambered heart. Birds and mammals have ...$D$... chambered heart.
Choose the correct option.

All the components of the conducting system can generate an action potential for the contraction of heart muscle,but the Sinoatrial node acts as the pacemaker because

The blood pumped by the $A$ ventricle enters the $B$ artery,whereas the $C$ ventricle pumps blood into the $D$. Choose the correct option for $A, B, C$ and $D$.

Which structure generates an action potential without any external stimulus?

The pulmonary artery carries: