When should an advertiser use an app extension instead of an app promotion ad?

When should an advertiser use an app extension instead of an app promotion ad?

• A) With keywords intended to direct users to the mobile website, but still give the option to download the app
• B) To run your ad on both Search and Display
• C) To re-engage with users who have already downloaded the app
• D) With keywords intended to drive app download, but still give the option to visit the mobile website