A) is wrong - those who're not going to get benefited by a proposal are not going to support it.
B) Govt revenues depend upon what the rate of the flat tax. False.
C) There's nothing in the passage to prove this. False.
D) This is proved in the passage, that the rate at higher income levels can sometimes be brought lower than that at the lower income levels. This still does not prove that the progressive tax system is totally illusionary - the person at higher income pays more tax still. For example, at income of $100,000, someone pays $20,000 as taxes annually, at 20%, while someone at $50000 income pays $12500 as taxes. at 25%.
E. The passage actually has the central point that the current system is not fair to the lower income people. The point about higher income ppl paying lower tax rate was brought in just to prove this, and to prove that the current, unfair system can lead to lower income person paying a higher tax rate than a person with higher income.
Thus the passage should logically terminate by advocating the newer flat tax system, rather than accusing the older tax system.
I'd go by E.
Who says elephants can't dance?