What is the correct sequence of processes occurring in sewage treatment plants?

• A
  Physical process $\to$ Aerobic process $\to$ Anaerobic process
• B
  Physical process $\to$ Anaerobic process $\to$ Aerobic process
• C
  Aerobic process $\to$ Physical process $\to$ Anaerobic process
• D
  Anaerobic process $\to$ Physical process $\to$ Aerobic process