When the average temperature of the earth's atmosphere is gradually increasing, that is called :

Options

A. CFC
B. Smog
C. Global warming
D. None of these

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks for the term used to describe the gradual increase in the average temperature of the Earth's atmosphere.

2. **Identifying the Cause:**
The increase in temperature is primarily due to the accumulation of greenhouse gases, such as carbon dioxide (CO2), in the atmosphere. These gases create a layer that traps heat.

3. **Explaining the Greenhouse Effect:**
The greenhouse gases allow sunlight to enter the Earth's atmosphere but prevent some of the heat from escaping back into space. This phenomenon is known as the greenhouse effect.

4. **Consequences of Temperature Increase:**
As the average temperature rises, it leads to various environmental issues, such as melting glaciers, rising sea levels, and increased frequency of extreme weather events.

5. **Defining the Term:**
The gradual increase in the Earth's average temperature due to the greenhouse effect is specifically referred to as "global warming."

6. **Selecting the Correct Option:**
Based on the explanation, the correct answer to the question is "global warming."

**Final Answer:**
The term used to describe the gradual increase in the average temperature of the Earth's atmosphere is **global warming**.

---