Consider the following four statements. Which might be an example of a positive network externality? Which might be an example of a negative network externality?
(i) People eat hot dogs because they like the taste, and hot dogs are filling.
(ii) As soon as Zack discovered that everybody else was eating hot dogs, he stopped buying them.
(iii) Sally wouldn't think of buying hot dogs until she realized that all her friends were eating them.
(iv) When personal income grew by 10 percent, hot dog sales fell.