Answer
c. both a and b
Work Step by Step
Birds and mammals have four-chambered hearts, which keep oxygen-rich and oxygen-poor blood completely separate. The pulmonary circuit takes oxygen-poor blood the heart had received from the tissues and pumps it to the lungs to pick up oxygen and then returns the now oxygen-rich blood to the heart. The systemic circuit takes the oxygen-rich blood that was returned to the heart from the lungs and pumps it to the tissues and then returns oxygen-poor blood picked up from the tissues back to the heart.
