4. We have a multiple regression model 0 1 1 2 2 3 1 2 y x x x x ? ? ? ? ? ? ? ? ? ? with 2 ? ? ~ (0, ) iid N , where y : annual salary, 1 x : education period, 2 x : gender. If it is male, 2 x ?1 . Otherwise, 2 x ? 0 . The OLS estimates for ?0 , ?1 , ?2 , and ?3 are 10, 0.3, 2.5, and -0.1, respectively. We want to make the regression models for male and female. (1) Write the regression model for male (ex: 1 y x ? ? ?8 0.2 ). (2) Write the regression model for female. (3) Explain the relation between education period and salary gain using the graph below. (4) Explain the relation between gender and salary gain using the graph below.