Mammals are characterised by

Options

A. Hair on the body
B. Mammary glands
C. External glands
D. All the above.

Answer

**Step-by-Step Text Solution:**

1. **Introduction to Mammals**: Mammals are a class of animals known for certain distinctive characteristics that set them apart from other animal classes.

2. **Hair on the Body**: One of the primary features of mammals is that their bodies are covered with hair or fur. This hair serves various functions, including insulation and protection.

3. **Mammary Glands**: Mammals possess mammary glands, which are specialized glands that produce milk to nourish their young. This is a defining characteristic of mammals.

4. **External Glands**: Mammals have various external glands, including sweat glands and sebaceous glands, which are found in the skin. These glands play roles in thermoregulation and skin lubrication.

5. **Four-Chambered Heart**: Mammals have a four-chambered heart, which allows for efficient separation of oxygenated and deoxygenated blood, supporting their warm-blooded nature.

6. **Internal Fertilization**: Fertilization in mammals is internal, meaning that the sperm fertilizes the egg within the female's body.

7. **Presence of Pinna**: Mammals typically have an external ear structure known as the pinna, which helps in sound localization.

8. **Eyelids**: Mammals have eyelids that protect their eyes and keep them moist.

9. **Warm-Blooded**: Mammals are warm-blooded (endothermic), meaning they can regulate their body temperature internally.

10. **Lungs for Respiration**: Mammals breathe using lungs, which are adapted for efficient gas exchange.

11. **Conclusion**: Given these characteristics, the options provided in the question can be evaluated. The correct answer is "all of the above," as all the mentioned features (hair on the body, mammary glands, and external glands) are indeed characteristic of mammals.

**Final Answer**: D) All of the above.

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