We might worry that there is something unobserved about the counties that is common in all of the districts within them. After all, Kern County is a rural, agricultural county, Los Angeles is urban and highly populated, and Sonoma is wealthy and largely high-wealth rural (it is California "wine country"). (a) (10 points) Run a regression of read on income, english, and studentTeacherRatio, but also include terms that control for anything that is common within all observations in a county - that is, include variables that account for county-level unobserved heterogeneity in read. Make sure you use heteroskedasticity-robust errors in your output