How does natural selection lead to the development of antibiotic-resistant bacteria?

This is super easy. In every population you have differences between the individuals in that population. If you apply a selection pressure to that population (like antibiotics to bacteria), you will kill all the bacteria that do not have the ability to withstand the drug. Thus, you have selected for those bacteria that happen to have a better ability to withstand it.

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.

it’s lava came from an explosive eruption