Answer to number 5 please

and explain your answer. 2 points 5. If a consumer can only buy combinations of pizza and beer, how do you define and/or describe the marginal rate of substitution of pizza for beer? [1 point] 6. What is an externality in microeconomics? Provide examples of, re- spectively, a positive and a negative externalities. [1 point] 7. Define public goods and explain briefly why they are regarded as market failures? [1 point]