A study in Ontario found that multisystemic therapy (MST) was not more effective at reducing ...

A study in Ontario found that multisystemic therapy (MST) was not more effective at reducing recidivism than providing usual services for high-risk youth. According to your text, which of the following statements was suggested as a potential explanation for this unexpected finding?
a. The study did not include a follow-up period.
b. The usual services group was at much lower risk to reoffend at the beginning of the study.
c. MST may have benefited youths on factors that were not included in the study.
d. The researchers failed to include a control group, so no explanation is possible.
e. all of the above