Fill in the blanks left in the classification of the fossil fuels.
| p | q | u | v | w | x | |
| A | Petroleum | Coal | Petroleum | Bitumen | Coal gas | Coke |
| B | Coal | Petroleum | Coal gas | Coke | Petroleum gas | Bitumen |
| C | Coal | Petroleum | Coal gas | Charcoal | Gasoline | Coaltar |
| D | Petroleum | Coal | Gasoline | Bitumen | Natural gas | Coke |