_____ hydrogen is used as a fuel in rockets.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question states that a specific type of hydrogen is used as a fuel in rockets. We need to identify that type.

2. **Identifying the Type of Hydrogen**: The type of hydrogen commonly used in rockets is "liquid hydrogen." This is because it is stored in a liquid state at very low temperatures, which allows it to be more compact and efficient as a fuel.

3. **Explaining Why Liquid Hydrogen is Used**: Liquid hydrogen is favored in rocket fuel for several reasons:
- **Low Molecular Weight**: This allows for a higher thrust-to-weight ratio.
...