You are interested in the effect of increased carbon dioxide versus normal air, and also in the effect of green light versus full sunlight on the growth of corn plants in a greenhouse. Although you can set up your experiment inside a greenhouse, it is possible that there will be plant growth effects due to effects that you do not know and may never know. Which of the following are important to ensure control of unknown variables?

A. An increase in carbon dioxide does not result in a substantial decrease of other necessary gases.
B. All seedlings are from one uniform strain.
C. The intensity or brightness of the green light equals the intensity of the full sunlight.
D. All temperatures and available water remain the same.
E. All of the choices are important.