All mammals , without any exception , are characterised by

Options

A. Viviparity and biconcave red blood cells
B. Extra - abdominal testes and a four chambered heart
C. Heterodont teeth and 12 pairs of cranial nerves
D. A muscular diaphragm and milk producing glands .

Answer

To answer the question "All mammals, without any exception, are characterised by," we will analyze each option provided and determine which one correctly represents a characteristic that is exclusive to all mammals.

### Step-by-Step Solution:

1. **Examine the First Option: Viviparity and Biconcave Red Blood Cells**
- Viviparity (giving birth to live young) is a characteristic of most mammals, but it does not apply to monotremes (like the platypus), which lay eggs. Therefore, this option has an exception.
- Additionally, not all mammals have biconcave red blood cells; for example, camels have ovaloid red blood cells. Thus, this option is incorrect.

2. **Examine the Second Option: Extra Abdominal Testes and a Four-Chambered Heart**
- While many mammals have extra abdominal testes, not all do; for instance, elephants do not have this feature. Hence, this part of the option has an exception.
- Furthermore, a four-chambered heart is not exclusive to mammals as it is also found in birds and some reptiles. Therefore, this option is incorrect.

3. **Examine the Third Option: Heterodont Teeth and Twelve Pairs of Cranial Nerves**
- Heterodont teeth (having different types of teeth) are found in many animals, not just mammals. Therefore, this characteristic is not exclusive to mammals.
- Similarly, twelve pairs of cranial nerves are also present in other vertebrates. Thus, this option is incorrect.

4. **Examine the Fourth Option: A Muscular Diaphragm and Milk Producing Glands**
- A muscular diaphragm is a unique feature of mammals that aids in respiration.
- Mammary glands, which produce milk, are also exclusive to mammals.
- Since both characteristics are found in all mammals without exception, this option is correct.

### Conclusion:
The correct answer is **a muscular diaphragm and milk producing glands**. These are the characteristics that define all mammals without any exceptions.

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