Normal cells are transformed into cancerous cells by:

Options

A. Plasmids
B. Viruses
C. Retroviruses
D. Bacteria

Answer

### Step-by-Step Solution:

1. **Understanding Normal and Cancerous Cells**:
- Normal cells have regulated growth and division, maintaining a balance within the body. They exhibit a property known as "contact inhibition," which prevents them from growing uncontrollably.
- Cancerous cells, on the other hand, show uncontrolled division and growth, leading to the formation of tumors.

2. **Identifying the Cause of Transformation**:
- The transformation of normal cells into cancerous cells can be caused by various factors, including genetic mutations, environmental factors, and infectious agents.

3. **Evaluating the Options**:
- **Option 1: Plasmids** - These are circular extra-chromosomal DNA found in bacteria and are not responsible for causing cancer in human cells.
- **Option 2: Viruses** - Certain viruses can lead to cancer by altering the genetic material of host cells.
- **Option 3: Retroviruses** - These are a specific type of virus that can convert RNA into DNA through a process called reverse transcription. They can activate oncogenes in the host's DNA, leading to uncontrolled cell division.
- **Option 4: Bacteria** - While some bacteria can cause diseases, they do not have the mechanism to transform normal cells into cancerous cells.

4. **Conclusion**:
- Among the options provided, retroviruses are responsible for the transformation of normal cells into cancerous cells by activating oncogenes.

### Final Answer:
Normal cells are transformed into cancerous cells primarily by **retroviruses**.

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