An example of secondary fuel is

Options

A. natural gas
B. coal
C. coke
D. petroleum

Answer

To determine an example of a secondary fuel, we need to understand the definitions and characteristics of primary and secondary fuels.

### Step-by-Step Solution:

1. **Define Fuels**:
- Fuels are substances that can be burned to produce energy. They can be classified into primary and secondary fuels.

2. **Identify Primary Fuels**:
- Primary fuels are those that are found in nature and can be used directly. Examples include natural gas, coal, and petroleum. These fuels are not extracted from other fuels; they are sourced directly from natural deposits.

3. **Define Secondary Fuels**:
- Secondary fuels are derived from primary fuels. They are extracted or processed from primary fuels and often require conversion or storage before use.

4. **Analyze the Options**:
- The options provided are natural gas, coal, coke, and petroleum.
- **Natural Gas**: A primary fuel, obtained directly from nature.
- **Coal**: Also a primary fuel, mined directly from the earth.
- **Coke**: A secondary fuel, produced by heating coal in the absence of air (a process called carbonization).
- **Petroleum**: Another primary fuel, extracted from the earth.

5. **Conclusion**:
- Among the given options, coke is the only secondary fuel because it is derived from coal through a processing method. Therefore, the correct answer is **coke**.

### Final Answer:
An example of a secondary fuel is **coke**.