From about 4 billion years ago to about 2.5 billion years ago, oxygen gas became more common in Earth's atmosphere. Oxygen was a toxic compound to most life forms from that time period. Today, oxygen is required by all but some of the most primitive life forms. Which statement best explains how early life forms that were harmed by oxygen evolved into today's life forms that require it?

littering streams           is the answer

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 correct answer is b