Lemme give it a shot
Approximately two hundred brands of personal computers are being manufactured, but we currently limit our inventory to only the eight most popular brands. We plan to increase greatly the number of computers we sell by expanding our inventory to include the ten best-selling brands.
Which of the following, if true, points out a major weakness of the plan above?
(B) The seven most popular brands of personal computers account for almost all computers sold.
--> The assumption is that by including more best-selling brands, they will increase the number of computer sold. However, if most of computer sold is from top 7, current 8 brands are sufficient... no need to go extra 2.
(C) As the users of personal computers become more sophisticated, they are more willing to buy less well-known brands of computers.
--> The passage says "best-selling" brands. (C) addresses "well-known"ness of a brand. I think you are connecting those two w/o knowing, thinking that if people buy less-known, then they won't buy well-known (best selling). The passage doesn't say anything about well-knownness so it's out of scope.