Assertion: Carbon dioxide controls the earth's heat balance.
Reason: Carbon dioxide has caused global warming.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Carbon dioxide controls the earth's heat balance." This means that carbon dioxide plays a crucial role in maintaining the temperature of the Earth by trapping heat in the atmosphere.

2. **Understanding the Reason**: The reason given is that "Carbon dioxide has caused global warming." This indicates that an increase in carbon dioxide levels leads to a rise in Earth's temperature, which is a phenomenon known as global warming.

3. **Analyzing the Relationship**:
- The assertion is correct because carbon dioxide, along with other greenhouse gases, helps to maintain a stable temperature on Earth by trapping heat. This is known as the greenhouse effect.
- The reason is also correct because excessive carbon dioxide in the atmosphere can lead to global warming by trapping more heat than necessary.

4. **Determining the Connection**: While both statements are true, the reason does not directly explain the assertion. The assertion talks about the role of carbon dioxide in maintaining heat balance, while the reason discusses the consequences of excess carbon dioxide (global warming).

5. **Conclusion**: Therefore, the correct relationship is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion. This leads us to conclude that the answer is option B: Both the assertion and reason are true, but the reason does not explain the assertion.