The contrast between the liveliness of the swans and human life is that the poet feels he has grown so old, that he is no longer young and active like the Swans. He feels Swans can move around with full of energy as they were some nineteen autumns back. The Swans still could go around carefree with no worries, but he feels it's no more the same with him. Thus, the poet relates himself to the swans and discerns how time has gradually decayed his youthful and robust body, while the swans are still the same, full of life.
A compound (A) with molecular formula $C_4H_9I$ which is a primary alkyl halide, reacts with alcoholic KOH to give compound (B). Compound (B) reacts with HI to give (C) which is an isomer of (A). When (A) reacts with Na metal in the presence of dry ether, it gives a compound (D), C8H18, which is different from the compound formed when n-butyl iodide reacts with sodium. Write the structures of A, (B), (C) and (D) when (A) reacts with alcoholic KOH.