在做逻辑问题,对疑惑选项进行取非很重要，但对于if从句的取非该如何取非呢。见大全-last-test2-19
19. How do the airlines expect to prevent commercial plane crashes? Studies have shown that pilot error contributes to two-thirds of all such crashes. To address this problem, the airlines have upgraded their training programs by increasing the hours of classroom instruction and emphasizing communication skills in the cockpit. But it is unrealistic to expect such measures to compensate for pilots’ lack of actual flying time. Therefore, the airlines should rethink their training approach to reducing commercial crashes.
Which one of the following is an assumption upon which the argument depends?
(A) Training programs can eliminate pilot errors.
(B) Commercial pilots routinely undergo additional training throughout their careers.
(C) The number of airline crashes will decrease if pilot training programs focus on increasing actual flying time.
(D) Lack of actual flying time is an important contributor to pilot error in commercial plane crashes.（D）
(E) Communication skills are not important to pilot training programs.
我在对c取非时，The number of airline crashes will not decrease if pilot training programs focus on increasing actual flying time.显然是不对的，后来我才明白应该The number of airline crashes will decrease if pilot training programs do not focus on increasing actual flying time.
可是我还是疑问，是否对于类似的if 从句都应该对if后面取非呢？或者是看不同情况而定，那么该怎么判断呢
请nn解答
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