In this excerpt from "ulysses" by alfred, lord tennyson, which of the following is a recurring concept?

i am a part of all that i have met;
yet all experience is an arch wherethro'
gleams that untravell'd world whose margin fades
for ever and forever when i move.
how dull it is to pause, to make an end,
to rust unburnish'd, not to shine in use!
as tho' to breathe were life! life piled on life
were all too little, and of one to me
little remains: but every hour is saved
from that eternal silence, something more,
a bringer of new things; and vile it were
for some three suns to store and hoard myself,
and this gray spirit yearning in desire
to follow knowledge like a sinking star,
beyond the utmost bound of human thought.

a)the heroic exploits of ulysses
b)the acceptance of circumstances created by fate
c)the knowledge of approaching death
d)the effects and difficulties of old age
e)the thirst for knowledge and new experiences