Determine whether the following statements are true, false, or uncertain.

1. Price elasticities of demand can never be constant along the demand curve.

2. Because of the law of demand, the income elasticity of demand is always negative.

3. Since for a company such as Spotify, the marginal cost of having one more customer is zero, the optimal price is equal to the average cost plus some markup which we obtain from what other firms do in the industry.

4. Game theory is only useful if every player has a "dominant strategy."

5. A profit-maximizing monopolist will always set prices so that the markup over total costs is larger than one in absolute value.

6. A profit-maximizing monopolist operates in the inelastic portion of the demand curve only when it produces a luxury good.

7. In general, the price elasticity of demand for an industry is smaller (in absolute terms) than the price elasticity of demand of firms within that industry.

8. If the marginal cost curve slopes upward, then we have to revise the marginal revenue equals marginal cost principle, and start considering fixed costs.

9. The average cost can never be equal to the marginal cost.

10. When a company produces a perishable good, such as yogurt, then the "marginal cost equals marginal revenue" principle has to be amended.