What is most likely the poets reason for choosing the rhyme scheme, which established throughout the poem, sonnet 29, suddenly changes just before the end

a) it establishes three stanzas as the verse, and the last two lines as the chorus of a song.

b) it reflects his own change from negativity to positivity.

c) to reinforce at the end the grim mood established in the beginning.

d) to contrast and emphasize the statement in the last two lines.