Please help!

@molinde, The issue with (A) is that is only refers to "some people.." whereas the argument says that these people do not "in general" gain weight, suggesting it is more of a rule than an exception that they do not gain weight. (A) explanation works only for "some", a portion of people that drink 2-3 beverages a day, and "some" usually implies less than "many" and the "majority" and can be as little as 1 person, hence what "some people" do is not helpful in explaining the general observation for this group. Let me know if this makes sense and if you have any further questions.