I believe the answer is:
1. they have to be extracted from the ground
(because they are made from the fossils of past organisms that buried well beneath the grown)
2. they emit carbon dioxide when burned to create electricity
( which is why these fuel tend to cause the most amount of pollution for the environment)
3. they are created through millions of years of pressure
(These resources is created from anaerobic pressure that happen for million years, which is why it considered as 'non-renewable', since people would not be able to replenish them in their lifetime)