Final answer:
The concentration of O2 at 12 degrees Celsius is higher in saltwater than in freshwater due to differences in solubility and water density.
Step-by-step explanation:
The concentration of O2 at 12 degrees Celsius is higher in saltwater than in freshwater. This is because the solubility of oxygen in water decreases as temperature increases. Additionally, the movement of liquid water is more sluggish, which can lead to localized areas with varying oxygen concentrations. In contrast, saltwater is denser than freshwater, allowing for better oxygen transfer and higher oxygen concentrations.