Which of the following are examples of perfectly competitive markets?

Joe likes peanut butter and jelly sandwiches for lunch. We can conclude that for Joe the cross elasticity of demand for peanut butter with respect to jelly is

A) positive because jelly and peanut butter are substitutes. B) positive because jelly and peanut butter are complements. C) negative because jelly and peanut butter are substitutes. D) negative because jelly and peanut butter are complements.

Assume that Juan concludes that he should eat less and play video games more. In this situation, it must be TRUE that the marginal utility of the last dollar spent on food is

A) more than the marginal utility of the last dollar spent on video games. B) less than the marginal utility of the last dollar spent on video games. C) equal to the marginal utility of the last dollar spent on video games. D) negative and less than the marginal utility of the last dollar spent on video games.