Here's a problem to solve: Carbon cycles through various reservoirs on Earth. The time it takes to form these reservoirs varies greatly. Which process in the carbon cycle takes millions of years to form the indicated reservoir?

Among the 4 following options, which is the correct answer?

- A: incorporation of carbon through digestion into animal tissues
 
- B: release of carbon into the atmosphere while breathing
 
- C: atmospheric carbon incorporated into sugars in plants
 
- D: breakdown of carbon in plant tissues to form oil
 