Assertion: Primitive atmosphere was of reducing type.
Reason: First hydrogen atoms combined with all oxygen.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that the primitive atmosphere was of reducing type. This means that the early Earth's atmosphere lacked free oxygen (O2) and was instead rich in reducing gases.

2. **Characteristics of the Primitive Atmosphere:**
- The primitive atmosphere was composed mainly of gases such as ammonia (NH3), methane (CH4), water vapor (H2O), and hydrogen (H2).
- The conditions on early Earth were unstable, with high temperatures and frequent volcanic activity, contributing to this unique atmospheric composition.

3. **Analyzing the Reason:**
- The reason provided states that "first hydrogen atoms combined with all oxygen." This refers to the process where hydrogen from the atmosphere combined with oxygen to form water vapor (H2O).
- This process is significant because it indicates that free oxygen was not present in the atmosphere; instead, it was bound in water molecules.

4. **Conclusion:**
- Both the assertion and the reason are correct. The primitive atmosphere was indeed of a reducing type, and the reason correctly explains that hydrogen combined with oxygen to form water vapor, thus depleting free oxygen in the atmosphere.

5. **Final Answer:**
- The correct answer is option A: Both the assertion and the reason are correct, and the reason correctly explains the assertion.

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