1. Comment on the correctness of the following statements. You must provide a full explanation of your reasoning, not just correct/incorrect. (a) (5 points) If an estimated regression is unbiased, it means that the estimated coefficient in the sample is equal to the true population coefficient. (b) (5 points) Heteroskedasticity leads to biased standard errors and coefficients. (c) (10 points) The validity of a difference-in-differences estimation does not depend on treat- ment and control groups being similar to each other. (d) (5 points) Omitted variable bias arises when we omit any variable that is important for explaining the y-variable