(1 point) What is the independent variable?

Bryan conducted an experiment with 4 bean plants. They were placed next to one another on a
sunny windowsill
. He gave each plant different types of water, ensuring to give them the same
amount. The plants were given tap water, distilled water, flavored water, and carbonated water.
Juan made the following observations after 3 weeks. Based on the results, Bryan concluded
that tap water is not good for plants. How can Bryan change his experimental design to improve
the validity of his results?