1. Gas Outlet 2. Manure 3. Slurry 4. Digester.
A-3 B-2 C-1 D-4
A-3 B-1 C-2 D-4
A-1 B-2 C-3 D-4
A-2 B-3 C-4 D-1
Thus, B is the answer.
1. gas outlet 2. manure 3. Slurry 4. digester.