如果lawmaker在制定税率以获得一定水平的收入时没有充分考虑到逃税造成的收入减少，才会任凭这个循环进行。换句话说，如果lawmaker制定税率时充分考虑到逃税会造成收入减少，则逃税不会导致调高税率（因为这个因素在制定税率时考虑进去了），则循环就不能进行。

遇到这种题型时可以从反面来想。比如题目要求非A来加强B，你说A会削弱B就可以了。这应该就是xdf著名的取非削弱理论。不知nn们怎么看。

题干Vicious cycle:
ppl evade taxable income------>(????)-------->lawmaker raise

income tax rates------->tax burden on nonevading taxpayers to

become heavier----->encourages even more taxpayers to evade

income taxes ------->law....

问题The vicious cycle described above could not result unless

which of the following were true?
逆否一下If the statement of the following were ture, then the vicious cycle described above will result. (既找假设)

就像Midfree大大说的那样 假设是连接reasioning line的一部分
而缺口就在reasioning line开始部分
为什么人们逃税就会-----导致------>lawmaker提高rates?
因为假设(即C中)lawmaker对revenue的确定是有一定要求的
C.When lawmakers establish income tax rates in order to generate a certain level of revenue, they do not allow adequately for revenue that will be lost through evasion.
lawmakers -----> certain level of revenue------>not allow evasion
所以
当ppl evade taxable income(也就是Vicious第一个条件开始成立的时候)
lawmakers才会

raise income tax rates------->tax burden on nonevading taxpayers to become heavier----->encourages even more taxpayers to evade income taxes ------->law....
这个恶性循环才会形成

希望大家指出错误 继续讨论...

你的逆否命题的推论有问题。The vicious cycle described above could not result unless which of the following were true并不等价于If the statement of the following were ture, then the vicious cycle described above will result.逆否命题并非单纯的顺序交换，而是因果关系的交换，你的这个推论，因果关系并没有变。你的转换就是A-B=非A-非B，这不是你否命题。