Macro is most likely to prefer soda because of the fact that he already has two candy bars. And it also gives 22 utils of happiness.
It is a common psychological trait in human beings that they prefer the things for themselves which they don't have currently. And if this act gives them happiness as well, then they are surly going to prefer it.
In this case, Macro already has candy bars, so he will prefer Soda which he hasn't with him at the moment.
