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.

cramming before a final is bad because it makes a student's stress level higher. students who embrace cramming to get through a final exam week find themselves struggling to perform consistently, once the brain adjusts to prolonged sleep deprivation.