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.

正确答案：The substitution effect increases the quantity of x by 90