Certain strains of E. coli bacteria can live in human intestines, where it relies on intestinal contents for nutrition. This bacterium also produces vitamin K, which humans use to produce blood clotting factors. In this case, the relationship between humans and E. coli is:

• A. Mutualism.
• B. Commensalism.
• C. Parasitism.
• D. Exploitation.

Answer: (A)

Verified step by step guidance

1. Identify the type of relationship by analyzing the benefits or harms to each organism involved.
2. Note that E. coli benefits by obtaining nutrition from the human intestines, so it gains from the relationship.
3. Recognize that humans benefit because E. coli produces vitamin K, which is essential for producing blood clotting factors.
4. Since both organisms benefit from this interaction, the relationship is not harmful or neutral to either party.
5. Conclude that the relationship is an example of mutualism, where both species benefit from living together.