In a study about the benefits of an online course in psychology, teachers prepared an online lesson in controlled experiments. Which of the following is a single-blind procedure?

1.Two groups of students sat in two different classrooms. Both groups were told they would learn a lesson in Psychology using online resources. A teacher in each classroom administered this quiz as a pre-test. Then the teacher pressed a control button to begin the lesson and stayed to observe how students used the online lesson. In one classroom, students studied this lesson. In the other classroom, there were videos of laboratory experiments. At the end of 45 minutes, this quiz was given as a post-test.
The researchers knew and the subjects did not know, who belonged to which group.

2.Two groups of students sat in one large classroom for a pre-test. The teachers told the students that one group was going to stay to study a course online, and one group was going to leave to conduct simple psychology experiments in a laboratory. After the pre-test, one group of students went to a laboratory, while the other group stayed in the classroom. 45 minutes later, the first group came back to the large classroom, and this quiz was administered as a post-test.

3.Two groups of students sat in the same classroom. Each student picked the group they would be in out of a hat. Half of the students just read the new lesson, while the other half of the students read the old lesson. They each new which lesson that they were given. They then took a quiz after as a post-test.

4.Two groups of students sat in two different classrooms. Both groups were told they would learn a lesson in Psychology using online resources. A teacher in each classroom administered this quiz as a pre-test. Then the teacher pressed a control button to begin the lesson and left the classroom. In one classroom, students studied this lesson. In the other classroom, there were videos of laboratory experiments. The teachers did not know which classroom had which lesson. At the end of 45 minutes, this quiz was given as a post-test.