one good way to avoid the internal-validity threat that might result from withholding the experimental treatment from the control group is to

a. give the experimental treatment to a sub-sample of the control group chosen by public lottery

b. identify volunteers who do not object to being in a no-treatment-control-group

c. give the experimental treatment to the control group after the experiment is concluded

d. work with the control group until all necessary data have been collected and then begin working with the experimental group