New forms of drug-resistant bacteria can evolve quickly because

The correct answer is - when a drug kills most of the bacteria, the ones left to breed are those that have a natural resistance to the drug.

Explanation:

When a drug or antibacterial medication used on the colony of a bacteria, the drug kills most of the specific bacteria from the host or culture, however, some bacteria remain to breed the next generation of bacteria as they had resistance to the particular drug.

The left bacteria breed and produce more resistant bacteria and after a few generations, a new form of drug-resistant bacteria develops. This follows the natural selection process and adapting the beneficial characteristic.

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