In Running an Experiment, It Is Crucial That There Exists

Multiple Choice

In running an experiment, it is crucial that there exists both subjects who receive the treatment, but also subjects who do not because:

A) without variation in the treatment, testing the hypothesis would not be feasible.
B) the treatment will be well calibrated.
C) this allows you to run regressions in the analysis step.
D) None of the answers is correct.