Which of the following meets the criteria for the absolute threshold of an auditory stimulus?

a. When exposed to 50 separate instances of the same very faint tone, Ronnie noticed it 25 times.
b. When exposed to a pair of tones 50 separate times, Keisha noticed a difference between them 25 times.
c. When exposed to a pair of tones 50 separate times, Tracy noticed a difference between them 50 times.
d. When exposed to 50 separate instances of the same very faint tone, Miguel noticed it 50 times.