An advantage of gas exchange in fresh water, compared with gas exchange in air, is that
a. water usually contains a higher concentration of oxygen than air
b. ventilation requires less energy in water
c. the respiratory surface does not have to be as extensive in water
d. water is easier to move over the respiratory surface
e. water loss through evaporation across the respiratory surface can be minimized

d. water is easier to move over the respiratory surface

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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c) 50% there are four chromosomes. after crossing over, 2 of the four have been changed. that means any gamete has a 2 in 4 chance or 50% chance of receiving the mutated chromosome.