_________is critical to performance-focused advertisers because they need to prove their advertising drives results.

Measurement is critical for performance-focused advertisers because it provides the data necessary to evaluate the effectiveness of their campaigns and demonstrate a return on investment (ROI). Without robust measurement, advertisers cannot determine which strategies are working, where to optimize spending, or how their advertising contributes to business goals like sales or leads. In-App Purchases are a specific type of conversion event that can be measured, but "measurement" is the overarching concept. Cost Per App Click (CPAC) is a metric used in measurement, but it represents only one aspect of performance and not the entire concept of proving results.