What is the freemium revenue$$ model?

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Part $1$

A$.$

When a publisher is paid whenever a visitor clicks on an ad and views an$$ advertiser's site

B$.$

When all of the services on a site are free to private$$ users, but there is a fee for business users

C$.$

When a social media business provides a basic service for free and charges fees for upgraded services

D$.$

When a publisher charges for products sold on its site

E$.$

When a user pays to gain access to any service on the site