In Shakespeare’s Sonnet 73, “That time of year thou mayst in me behold,” why is the speaker compared to the dying embers of a fire?

Ato highlight the intensity of his energy even as it shrinks with age

Bto show that he is on the verge of losing the vitality of his youth

Cto illustrate with compassion that he has all but lost the will to live

Dto show that the warmth he has to share with his beloved is humbler