Your text describes an experiment in which low-nutrient conditions select bacteria that would kill off other bacteria. When nutrients became plentiful again, these killer bacteria became less common because they had lower fitness in this environment, but they didn't become extinct. Why would some of the killer bacteria survive in the new environment?

A) They're waiting for a future time when that trait may be useful again.
B) They have changed the killer gene to a less costly one but can regenerate the killer gene when necessary.
C) Competition must be low enough that the disadvantage of having the killer trait isn't great enough to cause extinction.
D) The killer trait is a dominant trait, so it cannot be eliminated.