$A$ is a methane-rich fuel gas produced by $B$ breakdown with the help of $C$ bacteria. Here $A, B$,and $C$ refer to:

• A
  $A-$Gobar gas,$B-$aerobic,$C-$fermentative
• B
  $A-$Biogas,$B-$anaerobic,$C-$methanogenic
• C
  $A-$water gas,$B-$aerobic,$C-$methanogenic
• D
  $A-$Biogas,$B-$anaerobic,$C-$fermentative