Calvin and Michelle wanted to test the effects of different household cleansers on the growth of mildew. They selected three identical bathroom tiles to use in their experiment. To one tile, they applied Brand X cleanser. To another, they applied Brand Y cleanser. The third tile did not receive any
cleanser. They observed the growth of the mildew over a period of four days. Their results are shown
in the table below.

What is the independent, or manipulated, variable in this experiment?

A. the tile with no cleanser

B. the growth of mildew

C. the number of tiles used

D. the type of cleaner used