You want someone to write the whole 700-word essay?!
I would say the second option - "She skipped off, singing"
It gives a visual of her being very happy, instead of just saying she was happy.
Actually the real answer is A to explain the significance of the topic fro the United States.
<em>Robins are to birds as </em><em>dogs</em><em> are to </em><em>mammals</em><em>.</em>
robin: bird :: dog: mammal
The relation is one hyponym -- a word which is part of a bigger group sharing some characteristic -- and the superordinate -- the term that units all hyponyms.