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 different sizes.
Fill the pots completely with the same type of soil.
Plant tomato seeds in the pots of soil.
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?

The amount of fertilizer was not the controlled variable.
The amount of soil in the pots was not the controlled variable.
The increase in height of plants was the dependent variable.
The increase in height of plants was the independent variable.

A control variable is the one which does not receive any treatment in the experiment. Instead it is used to compare the changes in the experimental variable. The control variable remains unchanged in an experiment.

Among the given options, The amount of fertilizer was not the controlled variable. is the correct option. This is the flaw in the experiment because all the pots were filled with different amounts of fertilizers and no one pot was left without fertilizer. Therefore, a controlled variable was not used in an experiment.