Select the correct statement from the following.

• A
  Activated sludge sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria.
• B
  Biogas is produced by the activity of aerobic bacteria on animal wastes.
• C
  Methanobacterium is an aerobic bacterium found in rumen of cattle.
• D
  Biogas,commonly called gobar gas,is pure methane.