Briefly explain whether each of the following statements describes a change in supply or a change in quantity supplied.
a. To take advantage of high prices for snow shovels during a snowy winter, Alexander Shovels, Inc., decides to increase output.
b. The success of Nike and Adidas athletic shoes leads more firms to begin producing athletic shoes.

c. In the six months following the Japanese earthquake and tsunami in 2011, production of automobiles in Japan declined by 20 percent.