A phage is

Options

A. bacteria to be killed by a virus
B. virus which kills bacteria/fungi/algae
C. an association of copy DNA of retrovirus with host chromosome
D. a stage of division of virus inside host cell

Answer

**Step-by-Step Solution:**

1. **Understanding the Term "Phage":**
- A phage, specifically referred to as a bacteriophage, is a type of virus that specifically targets and infects bacteria.

2. **Identifying the Function of a Phage:**
- The primary function of a bacteriophage is to kill bacteria. It does this by attaching to the bacterial cell and injecting its genetic material into the host.

3. **Mechanism of Infection:**
- Once the bacteriophage attaches to the bacterial cell, it uses lytic enzymes (also known as lysins) located in its tail region to degrade the bacterial cell wall. This allows the phage to enter the bacterial cell.

4. **Replication Inside the Host:**
- After the phage has entered the bacterial cell, it can replicate using the bacterial machinery, leading to the production of new phage particles. Eventually, this process results in the lysis (breaking open) of the bacterial cell, releasing new phages to infect other bacteria.

5. **Conclusion:**
- Based on the information provided, the correct answer to the question "A phage is a" is: **"a virus which kills bacteria."**

**Final Answer:**
- A phage is a virus which kills bacteria.

---