I am currently practising some critical reasoning questions

I am currently practising some critical reasoning questions and I found one for which I answered wrongly and couldn't understand the answer at all. Hope someone else here has better luck!

Here it is:

As a promotional experiment, Omega Company distributes four million catalogs. In one of the two versions of the catalog, the description of each item for sale mentioned a "Made by Hand" label. The number of purchases from consumers receiving that catalog was twenty percent greater than the number of purchases from consumers receiving a catalog that did not mention the label. Thus, the mention of the label stimulated sales.

Which of the following, if true, could best be used to challenge the conclusion drawn above?

(A) Consumers receiving the catalog mentioning the label had previously purchased items from Omega Company by mail but consumers receiving the other catalog had not.

(B) Surveys showed that consumers returned purchased items to Omega Company during the promotional campaign at the same rate regardless of which catalog they received.

(C) The number of purchases from Omega Company declined substantially after the promotional campaign was completed.

(D) Omega Company mailed three times as many catalogs that did not mention the "Made by Hand" label as catalogs that did mention the label.

(E) Omega Company sold twenty percent fewer items during the year of the promotional campaign than it had during the previous year.