Aprofessor 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 samefinal exam, but some students have 90 minutes to complete the exam while others have 120 minutes. each student is randomly assigned one of the examinationtimes based on the flip of a coin. let denote the number of points scored on the exam by the student (0 100), let denote the amount of time that the student has to complete the exam ( = 90 or 120), and consider the regression modelyiith≤yi≤xixi , = ++yiβ0β1xiui= 0euiwhich of the following are true about the unobservable ? (check all that apply )uia. represents factors other than time that influence the student's performance on the exam. uib. will be zero for all students because time spent studying is likely the only factor that affects exam performance. uic. all students will necessarily have the same value of because they are part of the same population. uid. different students will have different values of because they have unobserved individual specific traits that affect exam performance.