A profesor decides to run an experiment to measure the effect of time pressure on final exam scores. He gives each of 400 students in his course the same final exam, but some students have 90 minutes to complete the exam, while others have 120 minutes. Each student is randomly assigned one of the examination times based on the flip of a coin. Let Yi denote the number of points scored on the exam by the ith student ( 0 less than or equl to Yi less than or equal to 100), let Xi denote the amount of time that the student has to complete the exam (Xi = 90 or 120), and consider the regression model Yi = β0 + β1Xi + ui,

E(ui) = 0 Which of the following are true about the unobservable ui?

a. there is no way to calculate it
b. ui represents factors other than time that influence the student's performance on the exam.
c. ui will be zero for all students because time spent studying is likely the only factor that affects exam performance.