This is an intermediary microeconomics question related to varian's textbook. Please give me some detailed explanation to each question. Thanks!

Define the terms "consumption externality" and "pure public good". In a many-person two-good economy, write down and briefly explain the conditions using the marginal rate of substitution (MRS) and the marginal rate of transformation (MRT) between the two goods for an efficient allocation in part (b), (c) and (d). (20 marks) (a) Define the above two terms. (4 marks) (b) Both goods are pure private goods. (4 marks) (c) Good 2 is a pure private good and consumption of good 1 by any individual imposes an externality on everyone else. (6 marks) (d) Good 2 is a pure private good and good 1 is a pure public good. (6 marks)