Plasmid has been used as vector because

Options

A. It is circular DNA which have capacity to join to eukaryotic DNA
B. It can move between prokaryotic and eukaryotic cells
C. both ends show replication
D. It has has antibiotic resistance gene

Explanation

### Step-by-Step Solution

1. **Understanding Plasmids**: Plasmids are small, circular pieces of DNA found in prokaryotic cells (like bacteria). They are separate from the chromosomal DNA and can replicate independently.

2. **Role as Vectors**: In biotechnology, plasmids are used as vectors to transfer genetic material into cells. This is crucial for gene cloning and recombinant DNA technology.

3. **Circular Structure**: One of the key features of plasmids is their circular structure. This circular form allows them to easily integrate with other DNA, including eukaryotic DNA.

4. **Integration with Eukaryotic DNA**: The ability of plasmids to join with eukaryotic DNA is significant because it allows scientists to insert genes of interest into eukaryotic cells, which can then express these genes.

5. **Antibiotic Resistance Gene**: While plasmids often carry antibiotic resistance genes, which help in selecting transformed cells, this is not the primary reason they are used as vectors. The main reason is their ability to integrate with eukaryotic DNA.

6. **Conclusion**: Therefore, the correct answer to the question "Plasmid has been used as vector because" is option (a): "it is circular DNA which has the capacity to join to eukaryotic DNA."