During primary treatment,all solids that settle form $A$ and the supernatant forms $B$.

• A
  $A$ - Primary sludge; $B$ - effluent
• B
  $A$ - Primary sludge; $B$ - secondary effluent
• C
  $A$ - Activated sludge; $B$ - clarified effluent
• D
  $A$ - Activated sludge; $B$ - effluent