Economics There are four statements below. Which one of them is true? a. The coefficient of determination (R2) decreases when an independent variable is added to a multiple regression model. b. A larger error variance makes it difficult to estimate the partial effect of any of the independent variables on the dependent variable. c. A larger variance for a coefficient {3-) means a more precise estimator. d. The key assumption forthe general multiple regression model is that all factors in the unobserved error term are correlated with the explanatory variables