Final answer:
Emily could likely hear the low humming noise because she has a lower threshold of hearing, which allows her to detect quieter sounds than Ryan can.
Step-by-step explanation:
Emily heard a low humming noise that her friend Ryan did not hear, likely because Emily has a lower threshold of hearing. This means she can detect sound waves that are fainter (or of lower intensity) than what Ryan can perceive. People's sensitivity to sound can vary; some individuals are able to hear very low-frequency sounds or very quiet sounds that others cannot. It is not related to faster reaction time or better ear protection, which affect other aspects of hearing. Neither is it related to higher frequency sensitivity, as high frequencies correspond with higher pitches rather than low humming noises.