Directions: Read the Assertion (A) and Reason (R) carefully and choose the correct alternative. Assertion (A): Carbon monoxide, when inhaled in sufficient quantity, can cause death. Reason (R): Carbon monoxide combines strongly with haemoglobin in the blood and prevents it from carrying oxygen.

Introduction / Context:
This question comes from basic human physiology and environmental science. It asks about the toxic effects of carbon monoxide and the mechanism by which it can cause death. Carbon monoxide is a colourless, odourless gas produced by incomplete combustion of carbon containing fuels. Understanding its action on haemoglobin is important for public health awareness and often appears in school level science exams.

Given Data / Assumptions:

• Assertion (A): Carbon monoxide when inhaled causes death.
• Reason (R): Carbon monoxide combines with haemoglobin.
• Haemoglobin is the oxygen carrying pigment present in red blood cells.
• We consider exposure to carbon monoxide at sufficiently high concentration and duration.

Concept / Approach:
Haemoglobin binds oxygen in the lungs and releases it in body tissues. It can also bind to other gases. Carbon monoxide has a much higher affinity for haemoglobin compared to oxygen. When a person inhales carbon monoxide, it competes with oxygen for binding sites on haemoglobin. If enough haemoglobin is occupied by carbon monoxide, the blood cannot carry adequate oxygen, leading to tissue hypoxia and possible death. Thus we need to check the truth of both statements and see whether the Reason provides the correct physiological mechanism.

Step-by-Step Solution:
Step 1: At low levels, carbon monoxide exposure can cause headache, dizziness, and weakness. At high levels, it can lead to unconsciousness and death. Therefore Assertion (A) that carbon monoxide when inhaled can cause death is true. Step 2: Reason (R) claims that carbon monoxide combines with haemoglobin. In reality, carbon monoxide binds to haemoglobin to form carboxyhaemoglobin. Step 3: The affinity of haemoglobin for carbon monoxide is many times greater than its affinity for oxygen. Once carbon monoxide occupies binding sites, oxygen cannot bind effectively. Step 4: As a result, even if the person breathes air that contains some oxygen, the blood cannot carry sufficient oxygen to body tissues. Step 5: This oxygen deprivation leads to damage in vital organs like the brain and heart and can cause death if exposure continues or is intense. Step 6: Thus Reason (R) is true and correctly explains the toxic effect described in Assertion (A).

Verification / Alternative check:
Medical and toxicology references explain carbon monoxide poisoning in exactly these terms: the gas binds to haemoglobin, forms carboxyhaemoglobin, and reduces oxygen carrying capacity of the blood. Symptoms and fatal outcomes are directly linked to the percentage of haemoglobin bound to carbon monoxide. This independent information matches the Reason and confirms that it is the explanation for the Assertion.

Why Other Options Are Wrong:
Any option that calls either Assertion (A) or Reason (R) false contradicts widely accepted medical knowledge. The idea that Reason (R) does not explain Assertion (A) is also incorrect, because the binding of carbon monoxide to haemoglobin is precisely the mechanism that leads to death in severe poisoning cases.

Common Pitfalls:
Some learners confuse carbon monoxide with carbon dioxide. Carbon dioxide at high levels can also be harmful, but the specific haemoglobin binding and carboxyhaemoglobin formation are distinctive features of carbon monoxide poisoning. Care must be taken to distinguish between these two gases.

Final Answer:
Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A), so the correct option is Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).