A. True
In an oligopoly market structure, there are a few large firms that dominate the market. The actions of any one participant in the market can have a significant impact on the marginal revenue of other participants. This is because the decisions made by one firm, such as changing prices or increasing production, can directly affect the market conditions and the demand faced by other firms in the oligopoly. As a result, the actions of one firm can influence the revenue and profitability of other firms in the market.