In the poem "Sailing to Byzantium" by William Butler Yeats, the speaker is frustrated by the cruelty of natural cycles, especially the progress from youth to old age, and he tries to initiate a new dynamic by leaving his homeland in search of spiritual rebirth in the holy city of Byzantium. Based on this synopsis, what theme is likely addressed throughout this poem?
A) heroism
B) love and duty
C) salvation
D) illusion and reality