If you examine a natural population of bacteria, it is common to find some antibiotic-sensitive and some antibiotic-resistant strains, even if the population has never been exposed to any antibiotic. Which explanation could account for this situation?

The resistant strain developed as a mutation and had no negative effect on fitness.

Explanation:

In examining a natural population of bacteria, resistant strain developed as a mutation which has no negative effect on fitness often causes observations of antibiotic-sensitive and some antibiotic-resistant strains, even if the population has never been exposed to any antibiotic.