We have a data set named "pp123" with the following variables, "highestgradecompleted", "earn," "age", and "agesquared."

Draft an R script with the following specifications (we aren't given the data file)

Regressing "earn" on "highestgradecompleted," "age," and "agesquared" such that it evaluates whether highestgradecompleted" has a discrete effect at particular thresholds: having 12 years of schooling ("highestgradecompleted" values equaling 12); having less than 12 ("highestgradecompleted" values < 12); and having more than 12 years of schooling ("highestgradecompleted" values > 12)