Which of the following best describes the occlusion effect as it relates to masking procedures?
a. The central nervous system's contribution to masking that increases thresholds in the presence of any masking
b. Enhancement of bone conducted sounds when the headphone is still in or on the ear can cause a false air bone gap in the low frequencies
c. When an increase in masking noise leads to over-masking
d. The amount of masking noise that is necessary to eliminate the non-test ear from participation