Which of the following is NOT an example of natural selection? A People selectively breeding pitbulls.
B Insect populations developing resistance to certain pesticides.
C Male birds of certain species developing colorful feathers to attract female mates.
D Bacteria becoming resistant to antibiotics.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

the answer is "a.) well–developed vascular system."

explanation: