Which of the following could act as a propellant for rockets ?

Options

A. Liquid oxygen + liquid argon
B. Liquid hydrogen + liquid oxygen
C. Liquid nitrogen + liquid oxygen
D. Liquid hydrogen + liquid nitrogen

Answer

To determine which of the given options could act as a propellant for rockets, we need to analyze each option based on the common knowledge of rocket propellants.

### Step-by-Step Solution:

1. **Understanding Rocket Propellants**: Rocket propellants typically consist of a fuel and an oxidizer. The oxidizer provides the necessary oxygen for combustion, while the fuel burns in the presence of the oxidizer to produce thrust.

2. **Analyzing the Options**:
- **Option 1: Liquid Oxygen + Liquid Alcohol**: Liquid oxygen is a common oxidizer, but liquid alcohol is not a typical fuel used in rocket propulsion.
- **Option 2: Liquid Hydrogen + Liquid Oxygen**: This combination is well-known in rocketry. Liquid hydrogen serves as a fuel, and liquid oxygen acts as the oxidizer. This combination is widely used in many space missions.
- **Option 3: Liquid Nitrogen + Liquid Oxygen**: Liquid nitrogen is not a fuel; it is often used as a cryogenic coolant but does not combust with liquid oxygen.
- **Option 4: Liquid Hydrogen + Liquid Nitrogen**: While liquid hydrogen is a fuel, liquid nitrogen does not act as an oxidizer and cannot support combustion.

3. **Conclusion**: The only viable option that consists of a fuel and an oxidizer suitable for rocket propulsion is **Option 2: Liquid Hydrogen + Liquid Oxygen**.

### Final Answer:
**The correct option is: Liquid Hydrogen + Liquid Oxygen.**

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