72. A chemist uses the standard addition method to determine the concentration of copper (Cu2+) in a river water sample to account for matrix interferences. Two 25.0 mL aliquots of the river water are processed as follows: Solution A: 25.0 mL of the sample is diluted to a final volume of 50.0 mL with distilled water. The measured absorbance is 0.25. Solution B: 25.0 mL of the sample plus 1.00 mL of a 500 ppm Cu2+ standard is diluted to a final volume of 50.0 mL. The measured absorbance is 0.50. Assuming the absorbance is directly proportional to the Cu2+ concentration, what is the concentration of Cu2+ in the original river water sample? (A) 5.0 ppm (B) 10.0 ppm (C) 20.0 ppm (D) 25.0 ppm (E) 40.0 ppm