We'd like to examine the effect of a new study regimen where students begin reading their text book for 30 minutes every day. To estimate a "first difference" with a regression we could measure students quiz scores before and after they implement the regimen and estimate a regression like: scoreᵢ= β₀ + β₁ afterᵢ + uᵢ, where "after" is a variable indicating a score the student registered after starting the new regimen. What is measured by β₁?
a. The average score before starting the regimen.
b. The average test score after beginning the regimen.
c. The average score.
d. The difference in scores after beginning the regimen.