Melissa used to buy 10 boxes of Brand A corn flakes per month each for $4 for a total of $40 per month. When the price of corn flakes increased to $5 per box, she reduced her monthly purchase to 8 boxes simply because it cost her more than $40 to buy 10 boxes as before. The reduction in Melissa's purchase of Brand A corn flakes because of this reason is called:
A. Substitution effect
B. Income effect
C. Quantity effect
D. Price effect
E. None of the above.