You are designing an ads bidding/optimization system where advertisers care about **CPA (cost per acquisition)**. Describe how you would design a system to **optimize toward a target CPA** while still participating in an auction. Cover: - What the objective is (platform vs advertiser) and how CPA is defined. - What models you need (e.g., CVR, value, conversion delay). - How to compute bids from predictions (e.g., bid shading, target CPA control). - How you handle delayed/attributed conversions, sparse data, and fraud. - Offline evaluation and online A/B testing metrics for CPA systems.