请教补充材料244

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.fficeffice" />

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.（A）

(E) Since 1987, sales of vacuum cleaners have risen twenty percent.

这题应该是要选“sales seminars有效”的答案吧，但是答案C好像是削弱阿，是我把题目理解错了，还是哪一步出了问题？