Which of these statements is correct? a. the amount of carbon on earth is essentially constant. b. the amount of carbon on earth fluctuates with the seasons. c. the amount of carbon on earth is increasing due to plant and animal growth. d. the amount of carbon on earth is decreasing due to consumption of carbon based fuels. e. both b and c are correct.

The amount is always constant. The answer is letter A. The carbon atom is recycle in the biosphere. It may take different forms but it is still the same carbon atom. Carbon in fuel may turn into carbon dioxide or carbon monoxide after it it burned. The CO2 may then be used up by the plants as fuel to make another form of carbon based compounds such as sugar (glucose). For more information, I suggest looking up the carbon cycle.