The E. coli benefit and humans are harmed. There is only a benefit for the humans. There is only a benefit for the E. coli. The E. coli and humans both benefit.
I gave my students this multiple choice question: The bacteria Escherichia coli (E. coli) live in the intestines of humans and make vitamins such as vitamin K and B-complex vitamins. Which would best describe the relationship between E. coli and humans?

Only one answer is correct among these 4 choices:
- The E. coli benefit and humans are harmed.
- There is only a benefit for the humans.
- There is only a benefit for the E. coli.
- The E. coli and humans both benefit.

Could you tell me which one is correct?

The E. coli and humans both benefit.