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. Which of the following is true?
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.