the main threat to the internal validity of a non-equalivent control group experiment is

a. the control group will fill resentful because control groups generally perform at a lower level on the pre-test than to do treatment groups

b. the possibility that observed differences between the experimental and control groups might reflect pre-existing differences between the groups rather than the treatment effect

c. differential attrition in the two groups across the duration of the experiment

d. all of the above