Why do technical writers provide captions for images?

Read the following excerpt from "dark tower" by claude mckay before you choose your answer. "we shall not always plant while others reap the golden increment of bursting fruit, nor always countenance, abject and mute, that lesser men should hold their brothers cheap; not everlastingly while others sleep shall we beguile their limbs with mellow flute, not always bend to some more subtle brute. we were not made eternally to weep. the night, whose sable breast relieves the stark, white stars, is no less lovely being dark; and there are buds that cannot bloom at all in light, but crumple, piteous, and fall. so in the dark we hid the heart that bleeds, and wait, and tend our agonizing needs." in context, the expression "the night, whose sable breast relieves the stark,/ white stars, is no less lovely being dark; " is best interpreted as a. the light of the stars overpowers the black of night b. the black of night overpowers the light of the stars c. black and white contribute equally to the beauty of the night sky d. black and white continuously compete for prominence in the night sky