PT72.S1.Q27-- RC weaken question-- Help!

Darklord

Hi,

So I am just really lost on why the right answer here was B instead of A. Can anyone explain why B is right and A is wrong?

When looking at the question, I focused primarily on the last two sentences of passage A (kinda treated those last two sentences as a LR question).

As a result, A looked like it weakened the argument passage A gives in these last two sentences because it created a reason for the phenomena (of rich people usually paying about the same under progressive tax as they would under flat tax) to be surprising (and thus less "unsurprising").

In addition, I just didn't see how B weakens the idea that this phenomena was "unsurprising", and as a result, I thought B was incorrect.

HELP

Thanks!