Network externalities: Select one: A. exist when the usefulness of a product increases with the number of consumers who use it B. are created when celebrity endorsements of products lead to a surge in the demand for those products C. can only exist when there are economies of scale D. prevent the dominance of a market by one firm.

A. )exist when the usefulness of a product increases with the number of consumers who use it

Explanation:

Network externality can be regarded as a change that occur in benefit as well as in surplus, which is been derived by agent from a good when there is a change in number of other agents that consumes this same type of good. Network externality can as well be regarded as "network effect" this effect is one is that is been had by one user of a good/service on the value of the product with respect to each other people.

It should be noted that Network externalities exist when the usefulness of a product increases with the number of consumers who use it