Answer:
Greg's and Allen's bodies metabolize the alcohol differently.
Explanation:
As the exercise suggests, the most likely reason for them to have two different reactions to the alcohol is that they metabolize the beer differently. Every individual could have different reactions depending their own context (for example if they ate or not before, if they are tired or stressed or not, etc.) but the most common explanation of this is that Allen drinks beer or alcohol more often than Greg and he developed tolerance to some of it's effects. Therefore, their metabolism works differently.
I would have to go with C or D. But I could not give a direct answer.
