Assertion : Carbon monoxide is a poisonous gas
Reason : Carbon monoxide combines with hameoglobin to form carboxy-haemoglobin which prevents absorption of oxygen by it.

Answer

To analyze the given statements regarding carbon monoxide (CO), we can break down the reasoning step by step.

### Step 1: Understanding the Assertion
The assertion states that "Carbon monoxide is a poisonous gas." This is true because carbon monoxide is known to be harmful to living organisms.

**Hint:** Consider the effects of carbon monoxide on living organisms and their respiratory systems.

### Step 2: Understanding the Reason
The reason provided is that "Carbon monoxide combines with hemoglobin to form carboxyhemoglobin which prevents absorption of oxygen by it." This statement is also true. Carbon monoxide has a very high affinity for hemoglobin, which means it binds to hemoglobin more readily than oxygen does.

**Hint:** Think about the role of hemoglobin in oxygen transport and how carbon monoxide interferes with this process.

### Step 3: Connecting the Assertion and Reason
Now, we need to connect the assertion and the reason. The reason explains why carbon monoxide is poisonous: it forms carboxyhemoglobin, which prevents hemoglobin from binding with oxygen. This lack of oxygen transport is what makes carbon monoxide dangerous.

**Hint:** Reflect on how the formation of carboxyhemoglobin directly impacts oxygen delivery to tissues.

### Step 4: Conclusion
Since both the assertion and the reason are true, and the reason correctly explains why the assertion is true, we can conclude that the correct answer is that both statements are true and the reason is the correct explanation of the assertion.

**Final Answer:** Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.