| A. Industrial nitrogen fixation B. Electrical nitrogen fixation C. Denitrification D. Ammonification |
| I | II | III | IV | |
| 1. | A | B | C | D |
| 2. | B | A | C | D |
| 3. | A | B | D | C |
| 4. | B | A | D | C |
| 1. Only I | 2. Only II |
| 3. Both I and II | 4. Neither I nor II |
| 1. Only I | 2. Only II |
| 3. Both I and II | 4. Neither I nor II |