Answer: Photosynthesis
Explanation: Photosynthesis removes CO2 from the atmosphere and replaces it with O2. Respiration takes O2 from the atmosphere and replaces it with CO2.
Stable elements are those which have atleast one isotope with no tendency to "decay", i.e., change into anotherelement.
The answer is Devonian.
Devonian is also known as the age of fishes.Devonian is named after the word Devon, a country somewhere south western England. This age of fishes began the domination of the fishes.