2. The following questions are unrelated to each other. Make sure to answer all parts of the question and use complete sentences. (2 points each) a) You have monthly data for a panel of workers on 1) whether they're in a union and 2) their hourly wage. You want to regress wage on union membership but are concerned about omitted variable bias. Name one omitted variable that will vary across workers but not across time and one variable that will vary across time but not across workers. Briefly explain your answers. b) What is the difference between perfect and imperfect multicollinearity? Give an example of each. c) Explain two issues that can come about when you do a time series regression. The issues should be specific and related to what I talked about in the lectures.