Final answer:
When a new firm enters a Cournot duopoly, the market price will depend on the new firm's production level and the existing firms' production levels. The market price could be below, equal to, or above $10 depending on specific market conditions. Therefore, none of the listed options is correct.
Step-by-step explanation:
In a Cournot duopoly, two firms compete by setting their quantities simultaneously. When a new firm enters the market, creating a triopoly, the market price will depend on the new firm's level of production and the production levels of the existing firms.
If the new firm enters the market and produces a quantity of 4,000 units, and the existing firms continue to produce 6,000 units, the total market supply would be 14,000 units. The new market price will be determined by the intersection of the market demand curve and the total supply curve.
It is not possible to determine the exact market price without additional information about the demand curve and the cost structures of the firms involved. Therefore, the correct answer is none of the listed options. The market price could be below, equal to, or above $10 depending on the specific conditions of the market.