What must a poem do to develop an extended metaphor?

Read the lines from "when i have fears" and answer the question. when i have fears that i may cease to be before my pen has glean'd my teeming brain, before high piled books, in charact'ry, hold like richgarners the full-ripen'd grain . . to fully understand the metaphor keats uses in these lines, readers must know that "garners" means the things that are harvested the places where harvests are stored the praises one receives for abundant harvests the people who own the harvests