Living in the atmosphere of CO is dangerous because:

CO, or carbon monoxide, is a colorless, odorless, and highly toxic gas that can be harmful to human health. When inhaled, it can have various negative effects on the body. Among the given options, the correct answer is option 'D', which states that CO combines with hemoglobin and makes it incapable of absorbing oxygen. Let's explore this in more detail:

1. Hemoglobin and its role:
- Hemoglobin is a protein found in red blood cells that is responsible for carrying oxygen from the lungs to the body's tissues.
- It binds with oxygen (O2) in the lungs and releases it to the cells and tissues throughout the body.

2. How CO affects hemoglobin:
- Carbon monoxide has a strong affinity for hemoglobin and can bind to it more tightly than oxygen.
- When a person inhales CO, it enters the bloodstream and binds with hemoglobin, forming a compound called carboxyhemoglobin (COHb).
- This binding is reversible but occurs much more readily than the binding of oxygen to hemoglobin.
- As a result, the presence of CO in the bloodstream reduces the amount of hemoglobin available for oxygen transport.

3. Reduced oxygen transport:
- When CO binds to hemoglobin, it forms a stable complex that prevents oxygen from binding to the hemoglobin molecule.
- This means that the oxygen-carrying capacity of the blood is significantly reduced.
- As a result, the body's cells and tissues receive less oxygen, leading to a condition known as hypoxia.

4. Health effects of CO exposure:
- The reduced oxygen transport caused by CO exposure can have serious health consequences.
- Mild exposure to CO can cause symptoms such as headache, dizziness, nausea, and fatigue.
- Higher levels of exposure can lead to more severe symptoms, including confusion, shortness of breath, chest pain, and loss of consciousness.
- Prolonged or high-level exposure to CO can be life-threatening, causing permanent damage to the brain, heart, and other vital organs, and can even result in death.

Conclusion:
Living in an atmosphere with high levels of CO is dangerous because it can combine with hemoglobin, forming carboxyhemoglobin and reducing the blood's ability to transport oxygen. This can lead to hypoxia and various health effects, ranging from mild symptoms to severe complications. It is crucial to prevent CO exposure by ensuring proper ventilation, avoiding the use of fuel-burning appliances in enclosed spaces, and regularly maintaining and inspecting such appliances.