OG-122 还是很困惑， 请NNs 帮忙

122.  When people evade income taxes by not declaring taxable income, a vicious cycle results. Tax evasion forces lawmakers to raise income tax rates, which causes the tax burden on nonevading taxpayers to become heavier. This, in turn, encourage even more taxpayers to evade income taxes by hiding taxable income.

The vicious cycle described above could not result unless which of the following were ture ?

答案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.

大家能不能给我讲讲 could not result unless 的逻辑关系。 我的理解是 答案C  整好导致 the vicious cycle could result . 怎么能选C 呢？？

lynnli , 我倒是理解 could not A unless B  ， 没有 B ， A 就不成立。  那是不是说 有B ， A就成立了。 回到此题， 我觉得答案C （ 我们所说的B） 在这里推不出 （ 我们所说的A ，The vicious cycle） 的成立呀。

还是我哪里的逻辑没绕过来？

设 the vicious circle result =A, the following is true =B
not A unless B 表示 A－＞B, 因此B是A的必要条件,A是B的充分条件(即,没B一定没有A,有B未必有A; 有A一定有B).

122逻辑推理: 纳税人逃税<==>lawmaker提高税率; 即,结论: 提高税率会导致更多人逃税.

从而形成恶性循环(vicious cycle); 这里最关键的是"纳税人逃税"和"lawmaker提高税率"之间的gap, 即"lawmaker提高税率"的原因: 逃税带来税收的减少. 而C选项正确实了这样的原因.

若没有考虑到此gap, 那就要理解C选项本身的前提: When lawmakers establish income tax rates in order to generate a certain level of revenue.该前提就是上述之gap: 逃税必定会带来税收收入的减少.

把C取非: lawmaker提高税率是为了保持税收收入不变(因为逃税必定会带来税收减少), 而他们却默许因逃税而损失税收. (allow for: to make it possible for something to happen or for someone to do something, especially something helpful or useful)

即C取非后不仅选项本生前后矛盾,而且也weaken了结论: 既然默许逃税损失税收,何必提高税率呢!(不提高税率,当然就没有本文的推理了).

个人理解,请讨论.