Question

new forms of drug - resistant bacteria can evolve quickly because
choose one option
a when a drug kills most of the bacteria, the ones left to breed are those that have a natural resistance to the drug.
b when scientists genetically engineer bacteria they use natural selection.
c scientists use extinction to selectively breed bacteria.
d a drug causes mutations in the bacteria, leaving only mutated bacteria left to breed.

Explanation:

Drug - resistant bacteria evolve based on natural selection. When a drug is applied, it acts as a selective pressure. Bacteria with natural resistance to the drug survive (as they are not killed by the drug), and these surviving bacteria then reproduce. Option B is incorrect because genetic engineering is a human - controlled process and not the natural way drug - resistant bacteria evolve. Option C is wrong as extinction is the complete disappearance of a species and not a method for bacteria to become drug - resistant. Option D is incorrect because drugs do not cause mutations; mutations are random events, and drugs just select for pre - existing resistant bacteria.

Answer:

A. when a drug kills most of the bacteria, the ones left to breed are those that have a natural resistance to the drug.