What do fish have to aid in oxygen exchange?
Options:
- gills
- bones
- tail
- no legs
- see work Let's think step by step:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Gills is respiratory organ of fish by which oxygen is extracted from water flowing over surfaces. So, the answer is gills.