蓝皮书上289题，请各位NN指点

Critics of sales seminars run by outside
consultants point out that since 1987, revenues of vacuum cleaner companies
whose employees attended consultant-let 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 id not send
employees to the seminars.

B.       
Vacuum cleaner companies that
have sent employees to dales seminars since 1987 experienced a great 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.       
Sine 1987, sales of vacuum
cleaners have risen twenty percent.

为什么选
        A？
        问题是对critics of sales seminars的削弱，A不是加强了吗？is the
most effective challenge应该说seminar 好才对啊？请各位大虾解释一二。

原文中说参加seminar，导致了公司sales low。我们现在可以想，在没有参加seminar的公司的sale和参加seminar的公司根本不在一个等级，比如一个是IBM，一个是清华同方。那么在seminar之后，IBM销售量还是大于清华同方。不就否认了是因为seminar的原因使得参加seminar的公司销售量不高啊！

我们如果在原题加一个assumption，表明在seminar之前，所有公司的销售量都一样，但是在seminar之后，参加seminar的公司销售量低了，那么才能说明是semina导致的销售量降低。