Two firms compete in the style of Cournot

Two firms compete in the style of Cournot. Both firms have a constant marginal cost. There are no fixed costs or capacity constraints for either firm. Which of the below can be a Nash equilibrium? Firm 1's market share: 80%, firm 2's market share: 20% firm, 1's Lerner index: 0.5, firm 2's Lerner index: 0.5. Firm 1's market share: 20%, firm 2's market share: 80% firm, 1's Lerner index: 2/3, firm 2's Lerner index: 1/6. Firm 1's market share: 80%, firm 2's market share: 20% firm, 1's Lerner index: 2/3, firm 2's Lerner index: 1/6. Firm 1's market share: 50%, firm 2's market share: 50% firm, 1's Lerner index: 2/3, firm 2's Lerner index: 1/6. Any of the above can be a Nash equilibrium depending on the cost and demand functions.