Context: The passage below describes an experiment. Read the passage and think about the variables that are described.

Dr. Hardin collected two species of algae, Chlorella rotunda and Tetraselmis cordiformis. He wanted to know whether one species released more oxygen as it grew than the other species.
To find out, Dr. Hardin prepared six culture tanks. He filled each tank with the same amount of a mixture of water and nutrients. Then, he added 1,000 live algae cells to each tank and sealed the tank. In three of the tanks, he added C. rotunda cells. In the other three, he added T. cordiformis cells. After five days, Dr. Hardin measured the amount of oxygen in each culture tank.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: growing algae in culture tanks.
Which of the following was a dependent variable in this experiment?
A. the amount of oxygen in the tanks
B. the species of algae
Pick the correct answer from the listed options and provide the letter of the selected option.
Answer: A