What is the under lying theme of this poem?

because i could not stop for death
by emily dickinson

because i could not stop for death –
he kindly stopped for me –
the carriage held but just ourselves –
and immortality.

we slowly drove – he knew no haste
and i had put away
my labor and my leisure too,
for his civility –

we passed the school, where children strove
at recess – in the ring –
we passed the fields of gazing grain –
we passed the setting sun –

or rather – he passed us –
the dews drew quivering and chill –
for only gossamer, my gown –
my tippet – only tulle –

we paused before a house that seemed
a swelling of the ground –
the roof was scarcely visible –
the cornice – in the ground –

since then – 'tis centuries – and yet
feels shorter than the day
i first surmised the horses' heads
were toward eternity –

a.
illusion
b.
rebirth
c.
salvation
d.
mortality