One student did an experiment to find if fertilizer can make plants grow taller. The steps of the experiment are listed below.
Take three pots of the same size.
Fill the pots completely with the same type of soil.
Plant tomato seeds in the first pot, orange seeds in the second pot, and bean seeds in the third pot.
Add different amounts of fertilizer to the three pots.
Keep the pots in the sunlight.
Add the same amount of water to the pots every day.
Measure the heights of the plants after regular intervals.

The results of the experiment were not valid. Which of these statements best describes a flaw in the experiment?

a. The increase in height of plants was the dependent variable.
b. The increase in height of plants was the independent variable.
c. The type of plants in the pots was not the controlled variable.
d. The amount of fertilizer was not the controlled variable.