In an experiment to measure the effect of fluoride varnish on the incidence of tooth cavities, 34 10-year-old-girls whose parents volunteered them for the study were randomly assigned to two groups. One group was given fluoride varnish annually for 4 years along with a standard dental hygiene regimen; the other group only followed the standard dental hygiene regimen. The mean number of cavities in the two groups was compared at the end of the treatments. Which of the following is a problem with this experiment?

a) Since the subjects were not randomly selected for the study, we cannot conclude that a reduction in cavities can be attributed to thefluoride treatment.

b) There was no control group.

c) The absence of replication increases the impact of random variation.

d) There was insufficient attention to the placebo effect.

e) The experimental subjects should not have been limited to girls.