Assertion : Smoking makes oxgyen less available to the body
Reason : In presence `CO2,O_2` binds less with haemoglobin.

Options

A. If both Assertion & Reason are True & the Reasonn is a correct explanation of the Assertion .
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Smoking makes oxygen less available to the body." This implies that smoking has a negative impact on the oxygen levels in the body.

2. **Analyzing the Reason**: The reason given is "In the presence of CO2, O2 binds less with hemoglobin." This suggests that an increase in carbon dioxide (CO2) levels affects the ability of oxygen (O2) to bind with hemoglobin, which is crucial for oxygen transport in the blood.

3. **Linking the Assertion and Reason**: When a person smokes, the inhalation of tobacco smoke increases the level of carbon dioxide in the body. High levels of CO2 lead to a condition known as hypercapnia, which can cause a decrease in the availability of oxygen.

4. **Understanding Hemoglobin's Binding Capacity**: Hemoglobin has a higher affinity for carbon dioxide compared to oxygen. The video mentions that the combining capacity of carbon dioxide is about 20 times greater than that of oxygen. This means that when both gases are present, hemoglobin will preferentially bind with CO2 over O2.

5. **Conclusion**: Since smoking increases CO2 levels in the body, it results in less O2 binding with hemoglobin, thereby making oxygen less available to the body. This confirms that both the assertion and the reason are true, and the reason correctly explains the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is a correct explanation for the assertion.

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