To see why (B) is correct, first look at the last paragraph of passage A. In it, the author argues that high-income earners avoid taxes more often in most developed countries (which have graduated tax codes) due to the complexity of tax law. But if this isn't because of the graduation, and instead because of other loopholes and special deductions, then it's not an argument against graduated tax codes per se.
(C) is wrong, on the other hand, because the author of passage B is in favor of taxing high-income earners at higher levels (see the second paragraph of passage B). As such, it's wouldn't make much sense for passage B's author to suddenly defend high-income earners. In addition, the argument presented in (C) doesn't address the larger issues of whether the arguments presented in passage A is right or not. Hope this helps!