In order to survive in the presence of the antibiotic tetracycline, some bacteria synthesize a protein called tetracycline-binding protein. As the name implies, this protein binds tetracycline. It is a monomeric protein with a single ligand-binding site. You have recently isolated a new pathogenic bacterium that is resistant to tetracycline, and purified the tetracycline-binding protein from this organism. You found that the Kd of the protein for tetracycline is 15 nM. At what concentration of tetracycline would the ratio of bound protein to free protein (i. e. [PL]/[P]) be 2