Dylan Is Interested in Finding Out How Sensitive His Roommate

Dylan is interested in finding out how sensitive his roommate is to sound. While he is watching TV, Dylan gradually turned up the volume on his stereo. Over the course of several days, Dylan did this a number of times and each time recorded the volume at which his roommate complained. This may be considered a study of _____, and the independent variable is the volume _____.

A) psychometrics; at which the roommate complains
B) psychometrics; on the stereo
C) psychophysics; at which the roommate complains
D) psychophysics; on the stereo