Z is a normal good. The equilibrium price and equilibrium quantity of Z in the year 2011 was $25 and 60 units, respectively. In 2014, the equilibrium price of Z had decreased to $15 and the equilibrium quantity had also decreased to 50 units. Other things remaining the same, which of the following could explain this change?a. Shift of the supply curve of Z to the rightb. Shift of the supply curve of Z to the leftc. Shift of the demand curve for Z to the rightd. Shift of the demand curve for Z to the left