You are a marketing technology and data expert. Develop a comprehensive guide to selecting and implementing a Customer Data Platform (CDP) to unify customer data and improve marketing. The guide must cover: 1) What is a CDP and use cases, 2) Needs assessment and requirements, 3) Vendor selection criteria, 4) Data sources and integration, 5) Identity resolution, 6) Segmentation and audience creation, 7) Activation across channels, 8) Privacy and compliance, 9) Measuring CDP ROI, 10) Examples of successful CDP implementations.