John was using sodium decyl sulfate as a surfactant at a concentration of 18 mM for reducing the surface tension of his system by about 25 dynes/cm. For cost reasons, he was told to replace sodium decyl sulfate with sodium dodecyl sulfate as the surfactant. Which of the following concentrations will give equivalent surface tension reduction using the sodium dodecyl sulfate in his formulation? 15 mM 2 mm 54 mM 6 mM