A professor decides to run an experiment to measure the effect of time pressure on final exam scores. He gives each of the 400 students in his course the same finalâ 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 studentâ (0<= Yi<= â100), let Xi denote the amount of time that the student has to complete the exam â( âXi = 90 orâ 120), and consider the regression model â, Which of the following are true about the un-observable â? â(Check all that applyâ)

a. Different students will have different values of because they have unobserved individual specific traits that affect exam performance.
b. represents factors other than time that influence theâ student's performance on the exam.
c. All students will necessarily have the same value of because they are part of the same population.
d. will be zero for all students because time spent studying is likely the only factor that affects exam performance