2. (5 points) Which of the following can cause OLS estimators to be biased? You must circle all answers and provide justification for our answers clearly. (a) A sample correlation coefficient of 0.99 between two independent variables both included in the model. (b) When one independent variable can be expressed as an exact linear function of two or more of the other independent vanables. (c) Omitting an important variable that is correlated with other independent variables in a model. (d) A sample correlation coefficient of 1.00 between two independent variables both included in the model