Besides Varying the Comparison Frequency with Respect to the Baseline

Besides varying the comparison frequency with respect to the baseline frequency, what other parameter is a valid measure of the subject being able to discriminate between presentation of the baseline and comparison frequencies?

A) Adding a visual stimulus between presentation of the baseline and that of the comparison frequencies
B) Adding an auditory stimulus between presentation of the baseline and that of the comparison frequencies
C) Varying the interval between presentation of the baseline and that of the comparison frequencies
D) Varying the intensities of the comparison frequencies
E) Varying the duration of the comparison frequencies