Answer:
a. Earth’s orbit and climate change
Explanation:
Over the years, the carbon cycle has seen changes in response to climate change. The amount of energy has varied in comparison to the changes is the earth’s orbit. The development of ice ages has seen a slowdown in the carbon cycle.
Similarly, temperature variations and climate changes initiated by the burning of fossil fuels have deeply impacted the carbon cycle by increasing the ratio of carbon dioxide in the atmosphere causing global warming. Therefore, the earth’s orbit and climate change are two recurring events in the Earth’s history that have impacted the carbon cycle.