ANS:B - 20% less

Sea water typically contains less dissolved oxygen compared to fresh river water. The main reason for this is that seawater has a higher salinity (salt content) than freshwater bodies such as rivers and lakes. The solubility of oxygen in water decreases as salinity increases. Additionally, various factors such as temperature, pressure, and biological activity can also affect the amount of dissolved oxygen in water. The correct comparison would be "sea water contains oxygen 20% less" as compared to fresh river water. This statement reflects the general trend observed in the oxygen content of seawater relative to freshwater bodies.