In recombinant DNA technology antibiotics are used:

1. to keep medium bacteria-free

2. to detect alien DNA

3. to impart disease-resistance to the host plant

4. as selectable markers

In Lederberg's replica plating experiment, what shall be used to obtain streptomycin resistant strain?
1.  Minimal medium and streptomycin
2.  Complete medium and streptomycin
3.  Only minimal medium
4.  Only complete medium