18. Critics of sales seminars run by outside consultants point out that

since 1987, revenues of vacuum cleaner companies whose employees attended

consultant-led seminars were lower than revenues of vacuum cleaner

companies whose employees did not attend such seminars. The critics charge

that for vacuum cleaner companies, the sales seminars are ill conceived

and a waste of money.
Which of the following, if true, is the most effective challenge to the

critics of sales seminars?
(A) Those vacuum cleaner companies whose sales were highest prior to 1987

are the only companies that did not send employees to the seminars.
(B) Vacuum cleaner companies that have sent employees to sales seminars

since 1987 experienced a greater drop in sales than they had prior to

1987.
(C) The cost of vacuum cleaner sales seminars run by outside consultants

has risen dramatically since 1987.
(D) The poor design of vacuum cleaner sales seminars is not the only

reason for their ineffectiveness.
(E) Since 1987, sales of vacuum cleaners have risen twenty percent.
请问答案A怎么解释? 怎么这么别扭,好像说倒了. 多谢各位ＮＮ！

原文的意思就是参加了培训的不如没有参加培训的，所以培训没用
找出一个削弱
a说的就是这个没有培训的厂呢以前就很赚钱，所以现在参加培训的厂呢没有它赚钱

也不奇怪，但是培训可能使得他们的差距缩小了啊，这就可以说明培训还是有用的，

这就是对原文的削弱了

我是这样想的，
原文推理是，
Evidence: 参加了的厂商盈利少了，
Conclusion: 所以参加这个会是没有用的。

A是说，那些没有参加的厂商，87年前盈利是最多的。意思就是说87年后盈利也少了。
所以不参加这个会，盈利也会减少。
所以参加不参加这个会，盈利都会减少。所以WEAKEN了其Evidence。

Those vacuum cleaner companies whose sales were highest prior to 1987

are the only companies that did not send employees to the seminars.
companies whose sales were highest prior to 1987 和 companies that did not

send employees to the seminars 之间在集合上是什么关系？这句话怎么翻译啊 ？