Private markets fail to reach a socially optimal equilibrium when positive externalities are present because the

a. private cost exceeds the private benefit at the private market solution.
b. social value exceeds the private value at the private market solution.
c. private cost exceeds the social benefit at the private market solution.
d. private benefit equals the social benefit at the private market solution.

Leading guitar string producer wound up inc. has enjoyed a competitive advantage based on its proprietary coating that gives its strings a clearer sound and longer lifespan than uncoated strings. one of wound up's competitors, however, has recently developed a similar coating using less expensive ingredients, which allows it to charge a lower price than wound up for similar-quality strings. wound up's competitive advantage is in danger due to a. a lack of perceived value b. a lack of organization c. direct imitation and substitution d. resource immobility