Define feasible, envelope and efficient portfolios, and describe the difference between a feasible set and an efficient set of portfolios?(2) What is a market portfolio? Why is it mean-variance efficient?

Hi thereI am looking for the answer for these two questions below , but please make sure  (a) Define feasible, envelope and efficient portfolios, and describe the difference between a feasible set and an efficient set of portfolios?(2) What is a market portfolio?  Why is it mean-variance efficient?