23. On a certain day, nine scheduled flights on Swift Airlines were canceled. Ordinarily, a cancellation is due to mechanical problems with the airplane scheduled for a certain flight. However, since it is unlikely that Swift would have the mechanical problems with more than one or two scheduled flights on a single day, some of the nine cancellations were probably due to something else.

The argument depends on which one of the following assumptions?

A.       More than one or two airplanes were scheduled for the nine canceled flights.

B.        Swift Airlines has fewer mechanical problems than do other airlines of the same size.

C.        Each of the canceled flights would have been longer than the average flight on Swift Airlines.

D.       Swift Airlines had never before canceled more than one or two scheduled flights on a single day.

E.        All of the airplanes scheduled for the canceled flights are based at the same airport.

参考答案：A 采用取非法。

思路：否定More than one or two airplanes were scheduled for the nine canceled flights.将削弱作者的结论。

看不懂啊，大牛帮忙解释一下好吗？ 怎么个取非呢？

不是大牛人的理解：
case：         On a certain day, nine scheduled flights on Swift Airlines were canceled

conclusion：some of the nine cancellations were probably due to something else
                  

evidence:     Ordinarily, a cancellation is due to mechanical problems with the airplane scheduled for a certain flight
                    it is unlikely that Swift would have the mechanical problems with more than one or two scheduled flights on a single day

Assumption: 如果没有more than one or two airplanes were scheduled for the nine flights, 那作者可能就会怀疑是机械问题造成航班的取 消。

我也来理解一把

原题就是说9个航班取消了，一般来说一天取消一两个由于机械问题，还可以理解，这么多的话，肯定有其它原因。

那推理的假设是什么呢? 看最后一句话，since it is unlikely that Swift would have the mechanical

problems with more than one or two scheduled flights on a single day, some of the nine

cancellations were probably due to something else.

Assumption就是前一半，对应下面的答案，就是A

此题并不难。
题干中提到，某天该航空公司取消了9个“航班”，但是，一天之内多架“飞机”产生机械故障的情况是不多见的，所以取消如此多航班一定有机械故障以外的原因。
此题关键在于搞清楚“航班”(scheduled flight)和“飞机”(airplane)的区别。
比如说我一天上海飞香港有10个“航班”，比如KA897,KA896, KA895....，但是可能是一架飞机（A320）来回飞的。

此题的assumption就在于这9个取消的航班不是一架飞机来回飞的。如果说一架飞机担任这9个航班的话，那么由于它的机械故障引起的9各航班同时取消是非常正常的事情，不用怀疑另有原因了。
所以答案就是选A
A.       More than one or two airplanes were scheduled for the nine canceled flights.