Which one of the following is the $CORRECT$ order of conversion of waste materials during biogas formation?

• A
  Monomers $\rightarrow$ polymers $\rightarrow$ methane $\rightarrow$ organic acids
• B
  Organic acids $\rightarrow$ methane $\rightarrow$ polymers $\rightarrow$ monomers
• C
  Methane $\rightarrow$ organic acids $\rightarrow$ polymers $\rightarrow$ monomers
• D
  Polymers $\rightarrow$ monomers $\rightarrow$ organic acids $\rightarrow$ methane