Among the Requirements Imposed on the Researcher Using the Before-After

Among the requirements imposed on the researcher using the before-after design with control group is the requirement that

A) the experimenter can arbitrarily assign test units to the experimental and control groups.
B) he will have to estimate the impact of history by means external to the experiment.
C) it is up to the test unit to self-select whether it will be a member of the control or experimental group.
D) each of the test units in both groups is measured before and after the introduction of the experimental stimulus to the experimental group.
E) the experimenter has to make an estimate of what the prior measurements should be.