Walt considers x and y to be perfect substitutes. They originally cost $10 and $9 respectively. His income is $720. One day the price of x drops to $8.
a. The income effect increases the quantity of y by 90.
b. The substitution effect increases the quantity of y by 80.
c. The substitution effect increases the quantity of x by 90.
d. The income effect increases the quantity of x by 80.
e. None of the above.