When Listening to a Tone Against a Background Noise, the Masking-Level

When listening to a tone against a background noise, the masking-level difference is the

A) amount to which low and high frequency tones are masked by the background noise.
B) difference in ability to detect high and low frequency tones.
C) difference in the ability to detect loud and soft tones.
D) difference in the threshold when the noise goes to one ear or both ears.
E) amount of background noise needed to mask a tone.