A student prepared four solutions of known [Cu2+] and measured the absorbance of each solution using the same cuvette. The graph shows the data for two absorbance measurements done for each solution. Which of the following identifies the most likely error that affected the absorbance recorded for the solution with [Cu2+]≈7×10−3M in the second trial?

a. The cuvette was rinsed with water between measurements.
b. A fingerprint was left on the side of the cuvette facing the detector.
c. The absorbance was measured at a wavelength where Cu2+ has a lower molar absorptivity.
d. The cuvette was not filled with the same volume of solution.