Fossil fuel A natural substance formed from the __ remains of ancient organisms that can be used as a source of __ Ex. Coal, oil, gas

## Step 1<br />The problem is asking for the type of substance that is formed from the remains of ancient organisms and can be used as a source of energy. The options provided are coal, oil, and gas.<br /><br />## Step 2<br />All these options are examples of fossil fuels. Fossil fuels are formed from the remains of ancient organisms that have been subjected to heat and pressure over millions of years.<br /><br />## Step 3<br />These fuels are called "fossil" fuels because they are formed from the fossilized remains of dead organisms. They are considered non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being made.