Answer:
d. water is easier to move over the respiratory surface
Step-by-step explanation:
The concentration of O2 in water is approximately 40 times less than air (4-8mL per liter), this fact in addition to water's greater density and viscosity in comparison with air, means a more demanding process in general. To adapt and reduce these disadvantages aquatic animals developed grills, creating foldings with a larger surface area for gas exchange. The main advantage for gas exchange in water is that the respiratory medium remains moist at all times.
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