Context: Read the passage. Then answer the question.

White sea urchins live in tropical oceans. Sea urchins are covered with hard, sharp spines. These spear-like spines protect the sea urchin from predators, such as fish. Sometimes a small worm, called a bristle worm, also uses the spines for protection.
The worm lives on the sea urchin, between the spines. To attack a worm on the sea urchin's body, a fish would have to get past the spines. When the worm lives on the sea urchin, the worm does not change the sea urchin's ability to survive.
Figure: a white sea urchin in the ocean.
Which type of relationship is formed when a bristle worm lives on a white sea urchin?
A. mutualistic
B. commensal
C. parasitic
Select the correct answer from the provided options and reply with the letter associated with it.
Answer: B