Aquatic animals breathe much faster than terrestrial animals because of the low amount of dissolved oxygen in water.

Option 1 is the correct answer because water contains significantly less dissolved oxygen than the air does. This means that aquatic animals have to work harder to extract oxygen from the water and obtain enough oxygen to meet their metabolic needs. As a result, they need to breathe at a faster rate to compensate for the low oxygen concentration in their environment.

Option 2, low amount of hemoglobin in their blood, is not the correct answer because the amount of hemoglobin in an animal`s blood does not directly affect its breathing rate. Hemoglobin is responsible for carrying oxygen within the blood, but it does not determine the rate at which an animal breathes.

Option 3, high metabolic rate of aquatic animals, also seems plausible, but it is not directly related to the reason why they breathe faster. While aquatic animals do tend to have higher metabolic rates compared to terrestrial animals, the primary reason for their increased breathing rate is the low level of dissolved oxygen in water.

Option 4, large amount of dissolved nitrogen in water, is also incorrect. Nitrogen does not play a direct role in the breathing process of aquatic animals. Oxygen is the essential gas that aquatic animals need