On the Google Display Network, your ad is eligible to show on a webpage if your:
Landing page matches that webpage’s content
Website matches that webpage’s content
Keywords match that webpage’s content
Ad text matches that webpage’s content
Explanation: Google’s system analyzes the content of each webpage to determine its central theme, which is then matched to your ad using your keywords and topic selections, your language and location targeting, a visitor’s recent browsing history, and other factors. Read more here
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