AHOD1721, Risk-based, Response-Adapted, Phase ll Open- Label Trail of Nivolumab+Brentuximab Vedotin(N+Bv) for Children,Adolescents, and Young Adults with Relapsed/ Refractory (R/R) CD30+ Classic Hodgkin Lymphoma (cHL) after Failure of First -Line Therapy, followed by Brentuximab+Bendamustine(Bv+B) for participants with a suboptimal response;CheckMate 744:Checkpoint Pathway and Nivolumab Clinical Trail Evaluation.