The motile bacteria are also to move by

Options

A. Flagella
B. Cilia
C. Pili
D. Fimbriae.

Answer

### Step-by-Step Solution

1. **Understanding Motility in Bacteria**:
- Motile bacteria are those that can move independently in their environment. This movement is crucial for their survival, allowing them to find nutrients, escape predators, and colonize new environments.

2. **Identifying Structures for Movement**:
- There are various structures that bacteria can use for movement. The question presents several options: flagella, cilia, pili, and fimbria.

3. **Analyzing Each Option**:
- **Flagella**: These are long, whip-like structures that extend from the surface of the bacteria. They rotate to propel the bacterium forward, making them a primary means of motility in many bacteria.
- **Cilia**: These are short hair-like structures found in eukaryotic cells, not prokaryotic bacteria. Therefore, this option can be eliminated.
- **Pili**: These are short, hair-like appendages that help bacteria attach to surfaces and other cells, but they do not aid in movement.
- **Fimbria**: Similar to pili, fimbriae are also involved in attachment rather than motility.

4. **Conclusion**:
- Since cilia are not present in bacteria and pili and fimbriae are not involved in movement, the only structure that facilitates motility in bacteria is the flagella. Thus, the correct answer is **flagella**.

### Final Answer:
The motile bacteria move by **flagella**.

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