取非削弱到底怎么取非？

都知道做假设题取非削弱是个好办法。可是其实到底怎么取非并不容易。不是简单的在谓语动词前加not。很多句子会包含多个否定词，或否定意味的词（e.g. unlikely, less ...)。比如 GWD3－32（附），请问取非到底应该遵循什么原则？

望牛人指点！！

Newspaper editorial:

In an attempt to reduce the crime rate, the governor is getting tough on criminals and making prison conditions harsher. Part of this effort has been to deny inmates the access they formerly had to college-level courses. However, this action is clearly counter to the governor’s ultimate goal, since after being released form prison, inmates who had taken such courses committed far fewer crimes overall than other inmates.

Which of the following is an assumption on which the argument depends?

A. Not being able to take college-level courses while in prison is unlikely to deter anyone from a crime that he or she might otherwise have committed.

B. Former inmates are no more likely to commit crimes than are members of the general population.

C. The group of inmates who chose to take college-level courses were not already less likely than other inmates to commit crimes after being released.

D. Taking high school level courses in prison has less effect on an inmate’s subsequent behavior than taking college-level courses does.

E. The governor’s ultimate goal actually is to gain popularity by convincing people that something effective is being done about crime.
 

简单点，取非就是在谓语前加上或去掉not呀

比如A取非就是Not being able to take college-level courses while in prison is likely to deter anyone from a crime that he or she might otherwise have committed.这就削弱了，所以A就是答案