Please choose the correct answer to the question: In a desert environment, cactus wrens often build their nests in cholla cacti to avoid predators. This behavior does not hurt the cacti. Which type of relationship do cactus wrens and cholla cacti demonstrate?

Answer1: competitive Answer2: commensalism Answer3: mutualism Answer4: parasitism

Answer format: answer1/answer2/answer3/answer4
Answer: answer2