You want to investigate the effect of GDP per capita (measured in thousands of dollars) on Life Expectancy (measured in years) and you decide to use a quadratic model. You estimate using data for a large number of countries and obtain the following results: According to your estimated relationship, the marginal effect of GDP per capita on Life Expectancy is positive (positive/negative/constant) up to a GDP per capita of 60 thousands of dollars (use an integer number in your answer), after which it becomes negative (positive/negative/constant). Note: in the first and last blank, choose only one of the options and make sure to use the correct spelling