A researcher plans to study the causal effect of police crime using data from a random sample of U.S. counties. He plans to regress the county's crime rate on the (per capita) size of the country's police force. Why is this regression likely to suffer from omitted variable bias? A. Other regressors are likely perfectly collinear with the (per capita) size of the county's police force, so they should be included in the regression B. There are other important determinants of a country's crime rate, including demographic characteristics of the population, that if left out of the regression C. There are other important determinants of a country's crime rate, including demographic characteristics of the population, that if left out of the regression D. None of the above are correct