Certain bacteria living in a human's large intestine help to produce vitamin K. This relationship is an example of:
A. mutualism.
B. commensalism.
C. plant parasitism.
D. animal parasitism.
Both organisms benefit from this relationship, which is why it is called mutualism. The human gets vitamin K and the bacteria get a place to live. Commensalism is when one organism benefits and the other is not affected. Parasitism is when one organism benefits at the other organisms expense.