The correct answer is (b), (d), (a), (c). Explanation & Key Takeaways: • The heart has two upper chambers—the left and right atriums—and two larger lower chambers—the left and right ventricles. A series of valves act like doorways to control blood flow in and out of these chambers. • The cardiac conduction system sends out electrical impulses to make the heart muscle contract and relax. Those pulses set the rate and rhythm of your heartbeat. • Here are the steps of blood flow through the heart and lungs: • The blood first enters the right atrium. • The blood then flows through the tricuspid valve into the right ventricle. • When the heart beats, the ventricle pushes blood through the pulmonic valve into the pulmonary artery. • The pulmonary artery carries blood to the lungs where it “picks up” oxygen. • It then leaves the lungs to return to the heart through the pulmonary vein. • The blood enters the left atrium. • It drops through the mitral valve into the left ventricle. • The left ventricle then pumps blood through the aortic valve and into the aorta. The aorta is the artery that feeds the rest of the body through a system of blood vessels. • Blood returns to the heart from the body via two large blood vessels called the superior vena cava and the inferior vena cava. This blood carries little oxygen, as it is returning from the body where oxygen was used. • The vena cavas pump blood into the right atrium and the cycle begins all over again. Hence option 3 is correct answer.