Where is oxygen content in a river or stream likely to be highest? a. the source b. the mouth c. the middle portion d. none of the above

Since the further the water goes down the more polluted and slow it becomes, then you could say that the closer to the source it is that it is cleaner and better. That's why the correct answer is A. the source. The source has the cleanest water that is full of oxygen and is great for any animal that might live there.