Final answer:
The correct answer is A, where drug resistance in bacteria evolves through natural selection. Resistant bacteria survive antibiotic treatment, reproduce, and pass on resistance traits, leading to a population resistant to the drug.
Step-by-step explanation:
New forms of drug-resistant bacteria can evolve quickly primarily due to natural selection. When an antibiotic is used, it usually kills most of the bacteria causing an infection. However, some bacteria may have mutations that provide resistance to the antibiotic. These resistant bacteria are not killed and can multiply, passing this trait on to their offspring, leading to a population of bacteria that the antibiotic no longer affects.
Therefore, the correct answer to why new forms of drug-resistant bacteria can evolve quickly is A: when a drug kills most of the bacteria, the ones left to breed are those that have a natural resistance to the drug. This process is an example of natural selection because the bacteria with the resistance trait are selected for by the environmental pressure exerted by the presence of the antibiotic.
Answer B, C, and D do not accurately describe the main process by which bacteria develop resistance to antibiotics. It is crucial to recognize that while mutations do occur in bacteria, the presence of an antibiotic does not cause these mutations; rather, it selects for bacteria that have pre-existing mutations that confer resistance. Incorrect use or overuse of antibiotics can exacerbate this problem by increasing the selective pressure that favors these resistant strains.