What theme is emphasized in this excerpt from "ode on a grecian urn" by john keats?

heard melodies are sweet, but those unheard
are sweeter; therefore, ye soft pipes, play on;
not to the sensual ear, but, more endear'd,
pipe to the spirit ditties of no tone:
fair youth, beneath the trees, thou canst not leave
thy song, nor ever can those trees be bare;
bold lover, never, never canst thou kiss,
though winning near the goal yet, do not grieve;
she cannot fade, though thou hast not thy bliss,
for ever wilt thou love, and she be fair!

a.
only things that continually change are true.
b.
unfulfilled desires and dreams seem sweeter.
c.
incidents and events depicted in art are true.
d.
art does not last forever; only memory does.
e.
incidents and events depicted in art are untrue.