The role of a just-noticeable difference best describes what is happening to Maria because as the sun is setting the room becomes darker and the light required to finish the painting would be affected requiring an introduction of an artificial light.
1.First notes above treble clef staff are G, A, B, C 2.First notes below treble staff are A, B, C, D 3.First notes above bass clef staff are B, C, D, E 4.First notes below bass clef staff are C, D, E, F