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: 2 LifeExpectancy = 62 +21,600GDPpc - 180GDPpc According to your estimated relationship, the marginal effect of GDP per capita on Life Expectancy is (positive/negative/constant) up to a GDP per capita of thousands of dollars (use an integer number in your answer), after which it becomes (positive/negative/constant). Note: in the first and last blank, choose only one of the options and make sure to use the correct spelling.