Ryan found that the amount of plant food remaining decreased an equal amount each day, and he used the entire 72 milliliters by the end of his study. Which statement is true about the relationship between the amount of plant food remaining and the number of days?

A. This relationship is a function because more than one amount of plant food remains each day.
B. This relationship is not a function because more than one amount of plant food remains each day.
C. This relationship is not a function because only one amount of plant food remains each day.
D. This relationship is a function because only one amount of plant food remains each day.