A company wants to study the effects that taking a vacation during different times of the year has on a worker's job performance upon returning. Each individual in the study takes a one-week vacation and upon returning, his or her job performance is monitored for one month. Which method of assigning subjects to groups is observational? :7 The control group consists of workers who choose to take vacation during the summer, and the experimental group consists of workers who choose to take vacation during the winter. :7 The control group consists of workers who are not assigned to be part of the study, and the experimental group consists of workers who are assigned to be part of the study. :7 The employees' names are alphabetized and numbered. The odd-numbered members are assigned to the control group and the even-numbered members are assigned to the experimental group. :7 None of these is observational