Free entry means that
a. the government pays any entry costs for individual firms.
b. government-funded research lowers the costs of patents and other barriers to entry.
c. a firm's marginal cost is zero.
d. no legal barriers prevent a firm from entering an industry.

c. a firm's marginal cost is zero.

Explanation:

Free entry is the condition in which new firms are allowed to enter an industry costlessly in order to achieve an economic benefit by establishing production and beginning to sell the product. To keep the equilibrium in the market free entry also allows free exit of firms.

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