Darren is studying the effect of increased carbon dioxide on plant growth. He sets up four tanks (terrariums). Each tank has the same type of plant, same potting soil, same amount of water, same amount of sunlight. The first tank has normal levels of Carbon dioxide. The second tank has 25% more carbon dioxide, the third 50% more and the fourth 100% more. Darren records plant height after each week for a four week period. What are the : IV, DV constant, control