The human ear's susceptibility to one sound will change due to the presence of another sound. The lowest value that can be heard by the human ear will increase with the appearance of another sound, this phenomenon is auditory masking. According to the above definition, which of the following is auditory masks? ['During the noisy class, the teacher must speak loudly so that the students can hear.', 'Wearing headphones for a long time to listen to music, you will feel that the volume of hearing gradually becomes smaller.', 'Humans cannot hear ultrasound from animals such as bats.', 'In the quiet room, we can hear the alarm clock ticking.']