Ecologists try to reduce the likelihood that variables not under the control of the experimenter will unduly influence the results of an experiment by
a. carefully setting up experiments so that the results are identical and do not need to be statistically analyzed to prove the result.
b. assigning treatments to subjects or plots methodically.
c. never performing the same experiment twice, but trying instead to improve each experiment.
d. replicating each treatment, including the control.