One Way to Avoid Confounding Results in an Experiment Is

One way to avoid confounding results in an experiment is to:

A) ensure the hypotheses are phrased as testable predictions.
B) test all participants in rooms with blue walls, regardless of their condition.
C) ensure all conditions are identical except for the independent variable of interest.
D) only ever test a maximum of six participants at a time, to allow ease of recording data.