A limitation of using historical mean returns to estimate the cost of common equity is that past performance may not accurately reflect future investor expectations or future market conditions. Historical averages are backward-looking measures. They summarize what returns were earned over a past period, but they do not directly account for changing economic conditions, shifts in interest rates, changes in business risk, new competition, or revised growth expectations. Because the cost of equity is a forward-looking required return, relying only on historical mean returns can produce misleading estimates if the future differs materially from the past. Choice C is correct because it identifies the main weakness: historical returns may ignore current market conditions and future prospects. Choice A is incorrect because historical returns are usually straightforward to calculate. Choice B describes a dividend-based model, not a historical-return approach. Choice D is also incorrect because the limitation is not that the method only applies to large firms. Financial managers often compare historical-return estimates with other methods, such as CAPM or dividend-growth approaches, to form a more balanced estimate of the cost of equity. Therefore, C is the correct answer.

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