The picture below shows an experiment to find out the highest frequencies which students can hear. When the teacher turns the dial the frequency from the loudspeaker increases. the students put their hands down when they can no longer hear the sound. Some students put their hands up while others have put their hands down. What conclusion can you draw from this observation?

The teacher suspects that some of the students are not giving honest responses, they are keeping their hands up even when they can no longer hear the sound. How could he check if he is right? *