Hydroponically Grown Plants Are Grown in a Nutrient Solution in Which

Hydroponically grown plants are grown in a nutrient solution in which air is bubbled. Why is bubbling of air through the solution needed?

A) Without the air, the solution would become so hot that the enzymes would denature.
B) Bubbling of air through the solution allows soil microbes to thrive.
C) Water and nutrient solutions don't have enough oxygen to sustain aerobic respiration.
D) The bubbling solution increases the relative humidity within the plant chamber which is needed for plant growth.
E) Bubbling of air in the solution is not really needed; it's just what is done.