Characterize each of the following as a positive-sum game, a zero-sum game, or a negative-sum game.
a. Office workers contribute $10 each to a pool of funds, and whoever best predicts the winners in a professional sports playoff wins the entire sum.
b. After three years of fighting with large losses of human lives and materiel, neither nation involved in a war is any closer to its objective than it was before the war began.
c. Two collectors who previously owned incomplete and nearly worthless sets of trading cards exchange several cards, and as a result both end up with completed sets with significant market value.