Dr. Brown wanted to test her cognitive therapy intervention for improving happiness in foster children. She gave a pre-test before and a post-test after her cognitive therapy intervention. While she was completing the therapy sessions, a new program began that gave all children in foster care supportive services and time at a summer camp. Dr. Brown found that children were happier at the post-test than the pre-test. Which of the following is true?

A. Dr. Brown's study is vulnerable to maturation threat.

B. Dr. Brown's study is vulnerable to a history threat

C. Dr. Brown may confidently conclude that her intervention worked,

D. Dr. Brown may be seeing regression to the mean.