A total of 200 10-year-old children of similar intellectual abilities participated in a noise experiment in their classrooms. They were tested for their ability to recall the meaning of a symbol in a textbook one week after the start of the experiment. Students did not know the meaning of the symbol prior to the experiment. The 200 students were split into 10 groups of 20 students and were taught the meaning of the symbol during a 15-minute lesson. Five groups learned the meaning of the symbol without any noise pollution outside of their classrooms. The other five groups learned the meaning of the symbol with a noise source of 55 decibels outside their classrooms, maintained continuously for the 15-minute lesson. One week after the lesson, 80% of the students who were not exposed to the noise sources were able to successfully recall the meaning of the symbol and 25% of the students exposed to the noise sources were able to successfully recall the meaning of the symbol.

what is the control?