They differ in their difference threshold. This is being defined when their sensory stimuli is different, making the individual perceive them also, as different. This occurs in the scenario given above for Susan couldn't hear the level 7 volume when Mary could already hear it.
The cover letter should be short and direct.
Answer: Andy's demand for beer to increase
Explanation:
Andy's views beer and pizza as complement to each other. Hence when the price of pizza decreases Andy's demand for beer would increase as he would order more beer than pizza so as to complement both offers.