For nonprofit grant eligibility, an analyst must answer a question about deciding whether a cost is an eligible expense. The available sources are: Source A - Application Portal: flags some budget lines as incomplete. Source B - Program Rules: define eligible expenses as direct service costs incurred during the grant period. Source C - Budget Narrative: explains why the cost supports the project. Source D - Reviewer Comment: asks for more detail about staff time. Which source should the analyst treat as controlling?