Carbon dioxide should be the answer considering its a huge focal point in our climate today and how it affects it.
Explanation:
Two important ways by which humans have affected the carbon cycle, especially in recent history, are: 1) the release of carbon dioxide into the atmosphere during the burning of fossil fuels, and 2) the clearing of trees and other plants (deforestation) that absorb carbon dioxide from the atmosphere
The elodea is getting the carbon dioxide from its environment like when underwater animals breathe out carbon dioxide the plants will use that carbon dioxide. The plants can also get the carbon dioxide from the atmosphere which can "dissolve" in water.
Answer: increased
Explanation: the more you have the better you can tell where something is acting on you, ie touch
