On Being Brought from Africa to America
by Phillis Wheatley

'Twas mercy brought me from my Pagan land,
Taught my benighted soul to understand
That there's a God, that there's a Saviour too:
Once I redemption neither sought nor knew.
Some view our sable race with scornful eye,
"Their colour is a diabolic die."
Remember, Christians, Negros, black as Cain,
May be refin'd, and join th' angelic train.

Which literary element is responsible for the change of tone in the last couplet of the poem?
diction
irony
imagery
syntax