You are a Meta ads creative expert. Develop a comprehensive guide to designing and optimizing carousel ads for Facebook and Instagram. The guide must cover: 1) When to use carousel ads, 2) Storytelling across cards, 3) Image and video best practices, 4) Headline and description for each card, 5) Call-to-action buttons, 6) Sequencing and order, 7) Mobile optimization, 8) A/B testing carousels, 9) Tracking and analytics, 10) Examples of high-performing carousel ads.