Reread lines 20-21. What is the meaning of these lines in terms
of Whitman's extended metaphor?

In "O Captain My Captain" by Whitman, The extended metaphor of the poem compares President Lincoln to a captain steering the "ship of state" which means guiding the union through the civil war.

Explanation:

An extended metaphor means; A metaphor that unfolds across multiple lines or paragraphs of a text, making use of multiple interrelated metaphors in an overarching one. The "captain' of the title however, becomes less essential to the progressive success and unity of the nation as it seemed in the beginning. because at first the "captain" (President Lincoln) is seen as entirely responsible for the safe return of the ship home.

But, the citizens continue to rejoice after their captain has fallen, they did not let the grief of the assassination on the president stop them from continuing their celebration as well as moving on with their lives.

well for some you have to do on your own on a paper but yeah

a. it reminds protestors of the reasons they have to protest.