Please determine whether the following statements are true, false, or uncertain.

1. If the cross-price elasticity of demand is negative, then the demand curve is not a good measure of willingness to pay.

2. Since marginal costs cannot be negative, a firm will never operate at the point on the demand curve where the price elasticity of demand is equal to -1.

3. The mark-up equation is only an approximation to the optimal pricing strategy. To define exactly what the optimal price should be, we need to know the complete cost structure of the firm.

4. The economics point of view is not valid for making decisions related to social undertakings, such as soup kitchens, or aid to poor families.

5. Indifference curves never cross. If they were to cross, preferences would be inconsistent.

6. If the marginal cost is equal to zero, then the optimal price will correspond point on the demand curve where the own price elasticity of demand is equal to -1.

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