How do some cells become brain cells and others become skin cells, when the DNA in all the cells is exactly the same?

the answer is:

carbon dioxide

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.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.