Francis is conducting an experiment to determine whether the acidity of ocean water affects the growth of ocean plants. He has 10 aquariums, each containing the same species of ocean plant. One has a pH of 8.2, the approximate pH of the ocean. The others vary in acidity from 0 to 14. Francis measures the growth of each plant after one month. Each aquarium is kept at the same temperature and receives the same amount of sunlight.

What is the variable being tested in this experiment?

acidity or pH
species of plant
temperature
amount of sunlight