When Ann went to her doctor, he gave her a hearing test. During the test, the doctor struck several tuning forks, each of which vibrated at a distinct pitch, and asked her to choose two tones that sounded almost the same in pitch. The doctor was testing Ann's ________.
◦ refractory threshold
◦ auditory convergence
◦ absolute threshold
◦ subliminal perception
◦ difference threshold