Bacteria are considered primitive organisms because they

Answer

**Step-by-Step Solution:**

1. **Understanding Bacteria as Primitive Organisms:**
- Bacteria are classified as primitive organisms primarily due to their cellular structure. They are prokaryotic, meaning they lack a defined nucleus and other membrane-bound organelles.

2. **Evaluating the Options:**
- **Option 1:** "Are small microscopic plants which cannot be seen by naked eyes."
- This statement is incorrect. Bacteria are not plants; they belong to a different kingdom (Monera) and are unicellular organisms.

- **Option 2:** "Causes serious disease in human beings, domesticated animals, and crop plants."
- While it is true that some bacteria can cause diseases, this characteristic does not define them as primitive organisms. Many organisms can cause diseases, but that does not classify them as primitive.

- **Option 3:** "Produce endospores which are very resistant to adverse conditions."
- This statement is correct. Certain bacteria can produce endospores, which are highly resistant structures that allow them to survive extreme environmental conditions. This ability is a significant factor in their classification as primitive organisms.

3. **Conclusion:**
- Based on the evaluation of the options, the correct answer is **Option 3**: Bacteria are considered primitive organisms because they produce endospores which are very resistant to adverse conditions.

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