The answer would be sections 2 and 3.
The author starts the poem conveying the beauty of nature and how it reminds him, at the same time, of the fleeting aspect of life. In the second stanza, he develops a sort of calculation of his own mortality and how close he is to the end. There's a strong irony here in how nature's grace and splendor serves to remind him of death. In the third and final stanza, he concludes that he should carry on living with intensity every moment as an antidote for life's brevity.
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