6. A set of five solutions are prepared by delivering 10 mL of unknown sample and increasing volumes of standard (0, 5, 10, 15, and 20 mL) into 50-mL volumetric flasks. Reagents are then added and diluted to volume. A plot of signals versus the final concentrations of standards is constructed. The x-intercept is −3.90 mM. What is the concentration of the unknown sample? (A) 0.39 mM (B) 3.9 mM (C) 19.5 mM (D) 39 mM (E) 195 mM