Jonathon was asked to investigate how the rising level of atmospheric carbon dioxide affects the growth rate of pine tree seedlings.

He potted four seedlings in identical pots and soil mixture and watered each one daily with exactly the same amount of water. He placed a clear glass dome over seedlings "A", "C" and"D". Seedling "A" was exposed to twice the normal levels of atmospheric carbon dioxide, while seedling "C" was exposed to half the normal levels. Seedling "D" was exposed to normal atmospheric concentrations of carbon dioxide while seedling "B" was left exposed to the atmosphere.

Which set up is not necessary for a fair test? Explain.