Read the following excerpt carefully before choosing your answer. This passage is from the end of then-Senator Barack Obama's November 2007 Speech on the "American Dream."
"There has been a lot of talk in this campaign about the politics of hope. But the politics of hope doesn't mean hoping that things come easy. It's a politics of believing in things unseen; of believing in what this country might be; and of standing up for that belief and fighting for it when it's hard. America is the sum of our dreams. And what binds us together, what makes us one American family, is that we stand up and fight for each other's dreams, that we reaffirm that fundamental belief "I am my brother's keeper, I am my sister's keeper" through our politics, our policies, and in our daily lives. It's time to do that once more. It's time to reclaim the American dream."

The purpose of placing the line, "It's a politics of believing in things unseen; of believing in what this country might be; and of standing up for that belief and fighting for it when it's hard," at the end of his speech is to:
A. Use denotative language to establish emotional distance and show his reserve
B. Encourage the audience to show its belief in America and what it could be
C. Offer examples and details to give his audience concrete support for his argument
D. Leave the audience with a lasting impression of his authority and credibility