关于逻辑的假设题探讨

各位大侠:
   我是刚接触GMAT的新手,正在上XDF的学习班,
   我想请教一下哦,演绎类的假设题有些什么好的做法分享呢?老师经常说,正确项取非可以否定原文但正确项如何取非呢,在哪里取的,动词,名词?
   some questions on assumption:
1)A company's personal director surveyed employees about their satisfation with the  company's system for awarding employee performance ratings.The survey data indicated that employees who received high ratings were very satified with the system.The personel director concluded from these data that the company's best-performing employees liked the system.
The personel director's conclusion assumes which of the following?
A)No other performance rating system is as good as the current sysytem..
B)The company's best-performing employees received high ratings.
C)Employees who receive low ratings were dissatisfied with the system.
D)Employees who received high ratings from a performance-rating system will like that sysytem.
E)The company's best-performing employees were motivated to perform well by the knowledge that they would receive performance ratings.
如何分析这道题,正确的是B,为什么D不可以呢?若用取非,D也可以否定原文呀?我觉得假设题真得很难,可以有什么方法让我掌握如何解题吗?