The opening of the excerpt "Soul, Penitent" from Hildegard's Play of Virtues is in which mode?

a) Dorian, beginning and cadencing on the note D
b) Mixolydian, beginning and cadencing on the note E
c) Phrygian, beginning and cadencing on the note F
d) Lydian, beginning and cadencing on the note G

Answer: a) Dorian, beginning and cadencing on the note D